Help! Could pressure build-up in tank cause the fuel pump to stay open

[Speeding Dakota]

Hello and Help! I'm at my wits end on this truck

Brief history:
I purchased a 1972 C20 that had been sitting for 8 years and after realizing the engine needed to be rebuilt, I pulled it out and swapped in a perfect running crate 350 engine & 400 trans. The 350/400 ran perfect in a 1980 C30 flatbed, so I figured it would be as easy as pie (was I wrong). Only thing that was removed was during the swap was the EGR/pollution stuff. And new gaskets were installed on the valve covers, fuel pump, thermostat, plugs and other basic tune up stuff.

The issue I'm have now (on the 4th carb I've tried) it starts to leak out of side or top seals like the floats are stuck open but at the same time it seems to build a lot of pressure in the gas tank. The reason I say that is the last time we started to replace the carb, gas just keep poring out of the line until someone removed the gas tank cap.

My question is: Is it possible the pressure inside the gas tank be causing the mechanical fuel pump to stay stuck open, which is forcing too much fuel into the carb?

Thanks for your assistance