carbs for a dual set up on a 318 poly wide block

A pair of 500 Carters/Edelbrocks is the best you can do in a small size. IIRC, most, if not all intakes will not take a Holley carb on top. If you can, then a pair of 390 Holleys is what I'd look at. OR a pair of (If you can actually find them) 400 carters.