I have a buddies 454 chev/omc engine out of his liberator in my garage,he bought my emi manifolds and riser,we are buying an rpm air gap intake but arent sure on a carb for it,any thoughts?It is a fairly new rebuild with a warrenty on it still,i think something in the 750cfm and mechanical secondaries.

A 750cfm mech secondaries is a good choice. I ran an 800cfm on my 454 and it was a little too big. Stock merc 454 and 502 mags use a 750cfm.