tell me what kind of carburetor should I buy

Sounds like your mind is already made up. Depends on your engine, cam, valves, intake, what you want to do with the car. For street driving plus performance, the "spread-bore" Thermoquad and Quadrajet are considered the best factory carbs ever made.

I have a 340 with an Edelbrock performer RPM intake and a mild cam. Don't really know the specs on it or the valves.