If it's an early Dominator bowl, the tail shaft bushing size is different, check that out first.
I contemplated the same thing.
There was no advantage to the early Dominator bowl, especially without a droop.

If it's a later Dominator bowl, that's different.

Early was the bud-mar casting I believe.

Flusher and Tom at JBP would know for sure.

Dan'l

It's a 1976 Eliminator 21' Day Cruiser.
Cruises at 3200, about 32 mph.
No need for a pontoon boat.
Built like a tank.

Dan'l

There may be more speed in a B impeller, but the juice isn't worth the squeeze, in my opinion.

My shitbox SBC will run miles and miles at WFO, and 55 plus mph.

For a max effort deal, and just for top end numbers, go with a B.
For an all around ski and family boat, I would go with the A. More grunt on the bottom end.

That's just my personal preference.

Dan'l

Dan'l

I'd call Tom at Jet Boat Performance. He would know.

Dan'l

Sounds like the older Dominator bowl with glacial bearing, (bushing) that is water lubed.

Try blasting the intake of the pump with another hose when firing it up on the hose.

Dan'l

It does sound like you have a A2 impeller.
My SBC, about 325 hp, turns an A impeller to 4600/4650 rpm.

Between the parts in your SBC, I'd think you'd be at a similar HP level.

As far as a 351 and 5300 rpm, either it's one stout 351, at about 475 HP, or it's a loose pump, (most likely)

Dan'l

From the front side.

Dan'l

I'd say they're half way while idling in the no wake zone, and under water when beached.

The exhaust hose is down hill to the transom, probably about 4 inches of elevation drop from the logs and snails.

Dan'l

Sounds like carburetor problems.

Start with the basics, a compression check.
If that is good, then it's sounds like carburetion.
Q-jets aren't your friend here. They belong on tow trucks and motorhomes. (In my honest opinion)
Tuning a q-jet can be done, but isn't as simple as a Holley.
Metering rods and and jets for a truck and usually too lean for a jet boat. The idle quality sounds like something is suspect. Q-jet should have a good idle, even on a jet boat.
Fuel pressure sounds like it's not steady at 7ish lbs to keep you running.
Non vented fuel tank(s) could cause this too, as well as a clogged fuel filter.

Several things you need to look into.
Make sure initial timing is good to go and your ignition system isn't failing either (bad coil or module?)

Dan'l

Just my $0.02.

As 76Manra455 said, something else is in the pump. Next thing is to pull the bowl off. The trailing edges of the impeller could be trashed, or even have a rock wedged in the impeller or bowl.

Pretty simple to get the bowl off, and if you find nothing in the pump, it's at least peace of mind that everything is ok.

Olds are not the best at oiling, if everything is still factory stock. I hope you don't have to get into the motor to find the vibration.
With the speeds vs rpm, it's most definitely in the pump.

Just my $0.02

Dan'l

It sounds like your idle air bleed adjustment is out of whack too.
Get the fuel bowl level addressed first and then your idle air bleeds.
You stated half a turn out, and as a starting point, should be a turn and a half out. Then adjust from there, in small increments.

Q-jet, 2bbl 2gc, Holley, Edelbrock, john Deere, Briggs and Stratton carbs, the starting point is a turn and a half out.

Hope this helps.

Dan'l

The swoop on the bottom side of the dash reminds me of a Hallett v drive.

The angled area just forward of the transom reminds me of early Eliminator jet boats.

Since it has a hin, it's newer than 72 or 73.

Does it have any strakes?

Dan'l