i have a 1986 pontiac trans am.i rebuilt the 305 ho that it had.i was tired of ever one telling me to use 350 i wanted to be different.i had it bored 30 over,stroked it to a 347,forged internals,doubel rolling timing chain,a pretty nice bump stick by crane very nice rough idel,chrome molley pushrods,comp valve springs,crane roller rockers,i had ported and polished stock heads,all new valves,shorty headers,edelbrock performer rpm intake manifold now my question what size carb u think i should use,it only my street car some thing for me to have fun with i dont care about gas milage.one more thing where can i find a 3.73 gear ratio for a 9 bolt differential thanks if u can help  

ok need more info like what is the lift and part number on the cam you have also what is the casting number of the heads you have and the valve size.now are the roller rockers 1.5 ratio or 1.6 ratio.  

Is the diff. a gm or ford never heard of a 9 bolt did you mean 9 inch rear.  

You want a 3.73 for a street car? May I recommend a 700-R4 transmission so you have a top speed higher than like 65mph? Seems like a lot of work for a street car. However to you question, You could get away with an Edelbrock 600-cfm, but until you post some cam specs and dynos I mean you might need something bigger. You got alot of work done to this thing. Maybe go tith 750-cfm or 800-cfm. That is a guess.  

Pontiacman:
Is the diff. a gm or ford never heard of a 9 bolt did you mean 9 inch rear.

It is a Borg Warner 9 bolt used in 80's Camaros and a few other cars.  

http://www.9bolt.com/  

PowerMax Hydraulic Flat Tappet Camshaft & Lifter Kit
Lift: .440''/.454''
Duration: 270°/276°
RPM Range: 1600-5400
part number-270-114132
the roller rockers are 1.6
the casting numbers are 14101081 intake 1.84 exhaust   1.5001  

For your carb, try this link. I've had pretty good luck with them in the past. http://www.carburetion.com/calc.asp
Remember that a carb is a precicely engineered vacuum control device. If the venturies are too big (i.e. too large of a carb) the engine will not be able to impose enough vacuum to the fuel circuit in the carb and thus will fall on it's face. A little too small of a carb will be a lot better than too big. Too big of a carb is one of the most common mistakes I see in my shop.  

I would use a holley 600CFM vacuum secondary carb. for that set up.