Fuel Issue. Carb?

[gable74]

72 K20
stock 350 internals
HEI
Electric fuel pump
Edelbrock 1400 carb

Truck sits a bit but usually has no issues. Last week I loaded her up to take a trip to the dump and as soon as I put it in gear it bogged down and ran like crap. Idles fine and revs fine in park/neutral.

I went for the easy stuff first.

Changed the plugs and wires (they were due anyway), and both looked fine.
Changed the fuel filter.
Checked vacuum - 18" once it was warmed up.
Timing was fine
Fuel pressure at the carb was about 5psi, maybe a hair less.

While doing some testing I found something interesting. If I turned the key on (don't start it) and let the fuel pump run for about 60 seconds, the truck would start and run perfect, drive perfect, and felt like normal. After about 30 sec it would fall on its face again and bog down in gear. I can do this repeatedly with the same results. I could replace the fuel pump and regulator (or install a stock mechanical pump) in case they aren't keeping up under a load, but I got a feeling its in the carb. Looking for second opinions.

Thanks,