If you're going to be running A 500cfm Holley 2bbl there is no need what so ever for the Magnum manifold whether it's from a 351E or any larger Magnum V6.
The discussion for using the larger Magnum manifold was for a 4bbl conversion
, furthermore adapters are no longer being manufactured to install a 500cfm carb on Magnum manifolds. Your present 305 manifold only requires a Mr. Gasket #1937 adapter to use the Holley carb, it's a simple bolt on installation.
