hi i need some help choosing a carb i have a 355 sbc flat top pistons vortec heads 280 magnum cam headers flowmaster exuast and summit intake manifold its in a 1983 camaro with the 700r4 auto trans i was looking a a holley 750 cfm with vac sec and a holley 650 dp i was wondering which one would be better for me or if i should stick with the 600cfm vac sec that i already have and build it up a lil bit thanks

 

I'd definitely go with a 750 with vacuum secondaries.

 

If it's mostly street driving under 5500 RPM, then you are giving up very little by keeping the 600 CFM carb.
If it is question for flow, then just hook a vacuum gauge up to your intake plenum area (manifold vacuum) and measure the vacuum at WOT. It should go to zero when you first hit the gas, then it may begin to climb a little. If it stays under 1" of vacuum through your second gear up until the point it shifts, then you aren't likely to gain much with the swap. If it goes over 1" for a good bit of your power range, then you have a significant restriction from the carb up.
If this is a drag only car, then I would consider a 650 or 750 DP.