what size carb do i need?

Holley's website recommends 750-850 cfm.

I entered the following data...

355 CID
7200 RPM
highly modified engine
double pumper, manual choke, for race.

With the amount of work you have into your motor, I would seek the advice of a carb expert, i.e. Barry Grant or maybe a forum member can make another recommendation.

http://www.holley.com/applications/CarburetorSelector/CarbSelection.asp