I have a stock 390 2 barrel,and would like to go to a 4 barrel what size carb should I go with? I plan on dual exhaust and headers at a later date. I have a 76 F250 with a factory 4 Speed

 

The factory used up to 600 CFM carbs and they work very well on a stock motor. Get vacuum secondaries.

 

I would probably opt for a little bigger like a 650 vac secondary myself but not any bigger (well maybe if you can get a deal on a 670 holley truck avenger, I haven't had any experience with them but have heard good things about them)

 

Go to the carb manufactuer's web sites. You will probably find a carb size calculator or a chart