In a book by Jason Scott, called How to Rebuild and Modify Your Muscle Car, there's a formula for how many cfm your engine should require from its carburetor:
carb cfm = ((engine cid x max rpm)/3,456) x volumetric efficiency
with this formula, you can calculate:
carb cfm = ((350 x 6,000)/3,456) x 0.80 = 486 cfm
My Quadrajet is rated at around 700 cfm, but it has vacuum secondaries. Vacuum secondaries only open as much as the engine needs them to open. Despite the formula, we can coax 'em open! I recommend that you use a carb with vacuum secondaries.
In the Winter 2002 issue of Hot Rod Engines, there's an article called The Other 350's - Pontiac. In this article, Jim Butler advises you to use 670 heads because the big 2.11 inch intake valves are set deep in the 72 cc closed chambers. In the same article, H-O Enterprises suggests '68 and '69 small valve heads off of 400's and 428's due to their small 72 cc combustion chambers and no valve shrouding with the small cylinder bore diameter. Finally, SD Performance recommends taking #47 or #11 castings, milling them .020", changing the intake valve seat angle to 30 degrees, mild intake side porting, and a 30 degree three-angle exhaust valve job. For more power, SD Performance says go to 2.05 intake valves and 1.71 inch exhaust valves. All three Pontiac experts in the article recommend less (advertised)cam duration than the Pontiac "S" cam. (288/302)
All I did was add a Quadrajet, slap on a set of big valve - small chamber heads, and shove in a mild cam that a friend didn't want. It's running great.
