1983 GMC pickup fuel problem

[GrymReaper]

Okay, I've got a 1983 GMC High Sierra C2500. I let some friends borrow it to move and of course it magically breaks down. They claimed it just stopped, I'm pretty sure they ran it out of gas, but refilling it didn't help. When I got it I was warned not to switch the tank from right to left for some undisclosed reason, I imagine they ran out of gas, tried to switch tanks, and then something happened. I just don't know what.
I was able to start the truck by filling the float bowl, but there was still no gas to the carburetor from the fuel pump. A pressure test on the car led me to believe the fuel pump might be at fault, but after removing it and performing a bench test with a jar of gas and some hose it seems to pump fine. I'm not sure how accurate such a test is, but I digress.
I replaced the fuel tank selector switch, but no matter what tank is selected the gas gauge is pegged at 3 o'clock. Am I looking at bad sending units here or what? If so, is there any way to test for that without dropping the tanks? Cause if I drop the damn things I'd rather it be for a real good reason, as I can't exactly drive the thing around until it's empty. Let me know what y'all think. This is the first time I've had to play with a fuel system south of the carburetor, so I'm kinda winging it.