according to mr grant=you carb your motor to a formula they provide when considering a carb=i will look for this info=was considering a 850 but was told this was to much=this is before alum heads were put on maybe now it will handle it-gettin 77 on this set up now

The formula for determining carburetor sizing is simple and straightforward:

Multiply: Displacement X Max RPM's X Volumetric Effeciency.  Then divide the result by 3456.  The answer yields the required carburator size.  Volumetric effeciency is rarely 100% in a normally aspirated engine.  Using a volumetric effeciency of 90% is more than generous for a mildly modified engine.

Thus:  460 X 5000 X .90 divided by 3456 = 599 cfm (rounded off).  You would do well with a 650 cfm carburator with vacuum secondaries.

So much for the belief a "750 double pumper" is the answer.

Dual Planes on big blocks tend to like more carb since only half the carb is working at any given time. A 750 CFM carb for example only allows the motor to see 375 CFM...way too small. I'd use an open hole spacer or increase the carb size to an 850 or 950. Sounds big, but properly jetted your motor will thank you.

A dual plane manifold is FAR superior in low and mid-range performance than a single plane manifold.  Plenum design in a dual plane enhances the velocity of the air/fuel mixture and that, in turn, assures maximum filling of the cylinders which is essential to throttle response..  Moreover, BOTH "halves" of the carburator are working all of the time.  The suggestion they are not is absurd.  Single plane manifolds are of benefit in the mid to upper rpm ranges...not the low to medium range.  Do not use an "open hole spacer" and do your best to avoid the age-old temptation to over-carb your engine.  You are operating in water, not on the dragstrip.  Bigger is NOT better.