best carb for 340?

You did not state which transmission you have but about any carb from 625-700 should run great on you combo.
The new Demon 625 works real smooth on my stock 318 truck, but for the 340 duster I am going to run a 700 DP with about the same motor combo as yours, but my car has a 4-speed and 4.30 gears.
Tune any of the above mentioned carbs, they will work.