Stock carb had two fuel supply lines from pump. One fed the float bowl and the other fed the accel pump well, then went to fuel return to the tank. Pump had a pressure regulator inside to regulate pressure on the float bowl line to limit pressure on the needle valve in the bowl.

I'd plumb in a regulator (3 hose type) after the pump, sending regulated pressure to the weber, and the bypass to the tank return line.

I suspect you are overpowering the float's pressure on the float valve and flooding the carb, leading to real bad mpg. Another possibility is a leaking float valve or misadjusted float (carb got dropped sometime??).

If the truck sat with stale fuel, I suspect clogged tank suction screen and fuel filter. Test fuel quantity flow over 10 seconds. I think you ought to see more than 8 ounces.

The choke is opening, right??