The typical crossover RPM between prepped ovals and squares is around 5800/6000. Below that ovals, above that squares make more power.
We built two identical 496s about 5 yrs ago for brothers. Comp solid roller cams: 246@.050 and .623 lift, performer airgap intakes, 850 holley dp, 9.6 compression, both sets pocket ported by me, 2.19/1.88 in both and unshrouded. 049s and 990s, its what they had. boats near identical 18 1/2 Kachina and Challenger, I built both pumps and did both set backs and set ups. both "A" impellers, both DPS inducers. both had Bassets and no mufflers. 049 boat: 73mph, 990 boat: 70, cut 990 impeller to "B" and they ran near identical, except it used more fuel, went BC on 990 boat and fuel was worse and gained only 1 mph. REMEMBER, these were 496s, we don't often bother with 454s.    TIMINATOR

By knowing the pressure at the various RPMs, you can figure the Gallons per minute flow at THAT RPM, and that changes with RPM/pump pressure.
Why does the engine block drain while sitting in the lake? Gravity. Water always seeks its own level.
The water jacket of the heads and almost always the entire engine block is above the level of the water outside the boat.
Water seeks its own level. The water in the engine will drain down to the level of the water outside, usually emptying most of the block, and all of the heads.
Before I moved to Phoenix, in the 1970s, we had a customer that didn't like the time that it took to refill the engine when started. So he added a check valve in the pressure line from the pump. He had almost immediate water flow upon start up, and I figured he had stumbled onto a good idea. I wondered why the boat companies didn't incorporate this feature. We got our answer next spring when the first time in the water he discovered that the block was cracked in several places on both sides in the lower water jacket area and the freeze plugs were severely corroded. It freezes back East. Lake water also contains no antifreeze/corrosion protection, so steel freeze plugs and iron blocks, and heads, whether iron or aluminum will  be corroded with water standing in it all winter.         
Water drains from the engine back into the pump, into the bowl, and out to the lake thru the impeller and suction housing.
Don't believe that? Remove the pump feed line to the engine and connect a garden hose to the pump itself, when you see water running from the pump you will be convinced.
It may take 3 or 4 minutes to drain the engine, but how long were you sitting in the lake putting out a skier, or B.S.ing about whose boat is faster before the time you actually raced? The heads empty first because they on top of the engine, but they are the hottest part of the engine and need the most cooling water, just sayin'.
When you put the boat on the trailer and take it out of the lake, did you ever notice how long it takes for the water to stop running out of the pump inlet? That's because the block and heads are emptying thru it. It takes just as long in the lake, but it does happen.
I did not expect this issue to become an encyclopedia, I thought everyone would have grasped these concepts much sooner, but since not everybody did, it just took a bit more explaining.   TIMINATOR

30 gallons per minute is a gallon every 2 seconds, thru a 3/8" hole.........    somebody take a garden hose which is 5/8" I.D, and standard municipal water pressure is 50-60 lbs, and see how long it takes to put a gallon in a bucket. Question everything, it makes you smarter.    just saying....   TIMINATOR

Block pressure will vary depending on: where the gauge is installed in relation to the dump line
                                                         what size dump line and how many of them
                                                         what is the dump line fitting outlet size
                                                         head gasket design I.E. car or marine
                                                         water feed line and "T" size
                                                         max engine RPM/HP (higher RPM means more bowl pressure)
                                                         nozzle size (changes bowl pressure)                 
                                                         bowl and impeller type (see above)
                                                         BB Chevys depend upon the year of the block, I.E. 1,2,or 3 center/lower coolant deck holes and     
                                                              Gen 4,5,or 6, they have different front and rear deck holes
                                                                        TIMINATOR

The Bassett "T" is a small on/off regulator, it keeps water out of the exhaust below about 1500-1800 RPM, that's normal. In the -8 fitting on the "spider" (that's the small lines that screw in the headers themselves) there is a double male that connects it to the feed hose. That fitting, if correct, has a divider in the center, and that divider has a hole from .080" to .120" diameter to regulate the amount of flow into the "spider". It sounds as if you either don't have that proper fitting, or the hole is too large. These exist because there is a great deal of difference in the pump pressure and block pressure found in jet boats. What was fine with your near stock engine will be too large when the new engine spins the pump faster and to a higher pressure.   
Always replace the calibrated spring and ball in the beginning of every season too, they rust and corrode.  Timinator

If you are satisfied with the way it runs and its not broken, leave it alone. Otherwise I have to know what the: boat, cam, intake, carb, exhaust, current valve sizes, porting if any, use of boat, and depth of your wallet is to give you good info. Heads could gain from 0 to 5 MPH depending on factors. Call me 623-877-8553

GT call me sometime and we'll talk, no hard feelings here. Just differences of opinion and those bring out better info for everyone.  I'll explain my thoughts on discussions too.     TIMINATOR

Cast cranks are not hardfaced, grind it to what it needs. Many bearings are supplied not only in .010, .020, and .030, but .040 and .060 also. Before cheap stroker cranks were common, we offset ground 460 Ford crank rod journals to use Pontiac rods (.250", yup that's a quarter inch undersize) to stroke them out to near 500", depending upon bore size. We used 403 Olds pistons and rings, that gave us a completely off the shelf 498" Ford. We had a ton of them in 4x4s and boats, they lived fine at over 500 HP. With custom oversize pistons they would be around 512". I suppose that we shouldn't be offset grinding 500 Cadillac crank rods down to BB Chevy size, (that's 3/8", yup .375" under) either. We have another one in here now that we are doing. Eagle rods and Ross pistons in this one. Incidentally, with the overbore this Caddy will be 548". And if we are the only people making this mistake can anyone account for the bazillion 383 Chevys out there with the mains cut from 400 to 350 size? As far as steel cranks go, other than the BBChevy 454 LS cranks from the 1970s (those had black counterweights from the Nitride hardening), I can't remember many factory steel cranks that were hardened at all.  TIMINATOR

Factory Q-jets on Olds 455s were 810 CFM stock! On cars! With smaller cams! Why go smaller on the carb? An 850 DP works great on a Crosswind dual plane intake for the Olds. I have a new one, and I have an Olds O4B intake too.  Start with the stock jets and change the power valve to a 45 (which actually starts the enrichment at 4.5" mercury. The Comp 280 is one of the better (but not the best) jetboat cam with log exhausts. TIMINATOR

Disconnect the driveshaft and run the engine, if no vibration fix the mount issue, if its still there the engine balance is wrong. At our shop we etch the balance number on the crank, flywheel and balancer, whether internally or externally balanced. Our balance card also indicates internal or external balance. Any good shop should do this and have the info immediately available by phone.  TIMINATOR

I have owned my own Race and performance business for over 40 years, written over 50-60 magazine articles for several Performance car and boat magazines, had a tech column, still write for Industry only magazines, not monthly newsstand stuff. I was guilty of oversimplification in some statements. There is no reason for me to banter semantics with anyone. Refer to my last post: "It must be worth my time."

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Max lift occurs when the piston is more than halfway down the cylinder. It does NOT cause piston/valve interference. Duration causes P/V interference during the overlap period when both valves are open. In a BBC the exhaust will clear near anything since its so far away from the piston crown. A tight lobe SEPARATION is just what a jet needs, it gives a narrow, peakier power band, but causes more interference problems. A jet only needs max power at max RPM of the pump capability (what the engine can actually turn it to). A BBC with the typical flat top piston with no intake notch, un milled heads, un milled deck will sometimes (50/50 chance, but will sometimes self clearance with a small crescent where the intake valve touches) clear the piston with a Comp Cams 292 hydraulic (great for Bassett exhaust, but too big for logs). A safer alternative and one that will always work is the Comp 288 or XE284 for Bassetts, and the XE 268 for log exhausts. Comp cams have typically faster ramp rates than other cams, this makes more power, but will cause a light valvetrain noise(ticking that you can't seem to get rid of, so don't try.) The best jet boat Comp solid roller is the 288AR, or for 496 builds if you want a bit more, the Comp XR286R. Any cam that says"easy on parts", or comes from a small or older "traditional" cam company usually means its ramp rate is slow, and may not make much more, or any more power than a fast ramp rate hydraulic cam(I.E. Comp Magnum or especially the Extreme energy series cams.) I have a cam analyzer and have analyzed thousands of cams in the last 25+ years. We charge  $18.50 for this service, and can save you much cash and time. Ramp rate is extremely important to power production, and all cam companies don't have the latest computer design or manufacturing equipment. You can't tell ramp rate, or quality by looking at a cam. It is a safe bet that buying the same cam from the same company that your dad/uncle/neighbor used "back in the day" will be a disappointment, BUT if you never compare it to something modern, you'll probably be satisfied. At least until you race somebody with a newer setup.....  Class dismissed.  TIMINATOR