I bought a 1980 project truck that someone had torn down, but ran out of patience before rebuilding. I've mounted a rebuilt TH350 transmission and 600cfm Edelbrock carburetor on the 305 motor and have the truck back on the road. New HEI distributor, wires, plugs, etc.
The problem I'm having is that when I am in gear (reverse or forward) and apply normal throttle, it behaves like it is flooding out and nearly stalls. A lite throttle press has no problems, and I can rev the engine aggressively all day long while sitting in Park. What are the top two or three things you would check and adjust first to resolve this issue.

 

New gas filter? Check.

(No, I mean literally, check it out. Make sure it's tight, not cracked or leaking,etc.)

 

vacuum leak can cause this issue along with improper timing