The factory aluminum manifolds for 305s (Monte Carlo SS for example) are for spreadbore carbs...CCC Quadrajets, in particular. Their secondary openings are too wide for an AFB-type carb (squarebore) and will require an adapter, just as a cast-iron manifold would.
If you DO go to an aluminum manifold, whether it's a GM or aftermarket, I strongly suggest some sort of heat insulator between the carb abd manifold. A wood, composite or phenolic 1/2 inch spacer will work great and makes a world of difference in preventing percolation.
A couple of other things I've run into regarding intake manifolds: if you're going to go with an Edelbrock (which is what many of the FLAPS carry) the Performer (2101) manifold is basically a stock replacement manifold. It flows about the same as a factory 4-barrel intake. The Performer RPM manifold (7101) is more of a performance oriented manifold with decent street performance. If you don't plan to run your motor over 4500 rpm or so, the 2101 manifold is fine.
Let us know how it turns out.
