Been posted a million times, but:
CFM = ((CI x RPM)/3456) x VE; VE for a stock engine is around 75% (optimistic).
This means for a square bore you don't need the secondaries until 4000 RPM, and you'd have to turn 8K to need more than 600 CFM
Another vote for a good overhaul on your existing Q-Jet. They're great carbs and typically just need a little TLC. The only problem is tuneability - there just isn't great availability on jets and rods any more
After the Q-Jet, any good vacuum secondary carb of 600-650 CFM is a good choice.