5/16" is the stock fuel line size in the 40's era. I don't think that you have a pump issue - I think that it is more likely an obstruction.

I am a fan of electric full pumps - but not for constant use. I have an Airtex in-line pump bolted to the frame in front of the passenger side rear-wheel. When it is 'off' the stock mechanical pump can pull fuel through it. I have the pump wired to a switch under the dash. When the car has sat for a few days the carb-bowls tend to go dry. With a flip of a switch the fuel system is primed in a just a few seconds. No cranking-cranking-cranking to refill the bowls. Once the engine is running the mechanical pump does the job and compatible with the fuel pressures expected from a carburetor.

The electric pump also makes a fairly annoying sound that I wouldn't want to listen to when cruising. But it is great in terms of hearing when the carb bowls fill and the needles close into the seats. The sound of the pump changes noticeably. Car is ready to start. The other benefit was at initial build/installation time. The electric pump can be turned on and off while leaks are detected an addressed - long before the engine is fired.

You mentioned going to a dual fuel tank set-up. Perhaps you should clean both tanks and invest in the permanent plumbing for that . . .