How many CFM for my 340?

The T-Q's came in two sizes, commonly rated @ 800 & 850.
A 750 would be very doable, however, I'd rather size the carb for the task at hand. Since you do not now the cam specs, what was the car built for?

Tell me your gear ratio and stall converter stall speed (if known)

FYI, (Not being a jerk about it, but...)

The Edelbrock carbs AFB's are 600 and 750
The AVS series are 650 and 800.

The smaller carb would be very happy on the street with gears numericaly at 3.55 and lower.
A 750 could go up top, but will work best with 373's and numericaly higher.