Choosing the right carb for my car (340 4-speed)

A double pump works best with a manual Trans or a pretty well build auto. I'd run a vacuum secondary or the AVS tuneable secondary air door. The AFB/AVS are mechanic secondary carbs with the ( IMO) crummy counter weighted doors (AFB) or the spring tensioned air door (excellent!)
The 650 AVS will be a nicely sprited carb on top of a dual plane (RPM) use it with something like a K&N extreme top filter.

The mystery is what cam you have.