After checking for fuel leaks, check float levels, needle/seat sealing, make sure float does not have a pin hole or crack letting fuel in and sinking the float, and power valves (a leaking/blown power valve will cause problems.) If you are using a mechanical fuel pump check fuel levels with the engine at a higher than idle RPM. I watched a Chevelle on the chassis dyno that seemed to run/idle fine, then under acceleration it was blowing fuel out the carb vents.

Anyhow a 750 carb is a good size for performance, but a lower RPM street driving you will be in the transition circuit area alot. This area can be adjusted by changing the idle restriction in the metering block, but a smaller carb will put the carb into the main metering circuit sooner because it will take a larger throttle angle to cruse at the same speed.