If you notice RPM style intakes do not have a Spread bore/Quadrajet base it is because it would not be useful in terms of flow,the small primaries need smaller ports to function properly and keep velocity up.You might look at the Edelbrock Performer Air-Gap #2601 which has a multi-fit base.One manifold that is overlooked since they don't advertise is the Offenhauser Dual-Port which has isolated runners for primary and secondaries check out their catolog at Exceter Auto online and read up on it,by speeding up velocity at lower speeds they are more fuel efficent when the larger primaries open the velocity picks up the incoming extra fuel along for the ride.The Spread bore manifolds are better suited for this than the square bore intakes.I have ran these with a Holley 650 Cfm Spreadbore carb that replaces the Quadrajet and was very satisfied with performance and mileage.If a quadrajet is in good shape it may be OK but tuning it and finding parts not too EZ.