More fuel problems

I'm still not getting fuel. So far: new fuel pump, new filter, new fuel lines from carb all the way back to the tank. Truck runs for a minute if I pour gas directly into the carb.

Today I rigged up a piece of rubber fuel line, to a barbed hose adaptor, to a 1/4" air line out from under the truck, to a brass valve I had in my garage, back to my compressor, which I set at 15 psi. I hose clamped the fuel line to the gas tank pickup, opened the gas cap and charged the air line up to the gate valve. Then I sat next to the gas fill so I could hear if it bubbled when I put the air to it. When I opened the valve and listened to the tank, I expected either bubbling or to hear the sock get blown off. Instead, I heard air blowing straight into the tank and got some fumes in the face. I thought maybe that I had goofed and just had an empty tank, so I added a couple gallons of gas to it. I put some more air into it, and still got a blast of air, no bubbling.

I'm concerned that this means that the pickup tube has rusted off and fallen inside the tank, thus no longer submerged in gas. Has this happened to anyone? Is there a remedy besides replacing the tank?

I think you've figured it out - the pickup isn't going down into the gas.  And that was good detective work.

But you don't have to change the tank.  The sending unit isn't that hard to replace.  You can usually lower the tank enough with the all-thread they hang from to get to the retainer, which you have to turn with a brass drift and a hammer.  Then you can pull the sender out.

And you can get the right part number on the page at Documentation/Fuel Systems/Fuel System Part #'s/Fuel Senders and Pumps.

Personally, I'd leave the piece that apparently fell off in the bottom of the tank.  I don't think it is going to hurt anything.

Honestly I think I might replace the tank anyways, it's got a hole near the top somewhere. From what I can see, it wouldn't be too difficult to drop it, correct? The two pieces of allthread, disconnect the sending unit and filler neck, and just lower it down yeah?

That's pretty much all there is to it, although the devil is in the details.

It's been about a year and a half since I last dropped my tank, I described the process I went through here: http://forum.garysgaragemahal.com/How-do-I-empty-my-fuel-tank-td19972.html

Yes, read Matthew's account.  And be aware that a little bit of gas in a tank makes it very unwieldy.  VERY.

But a new tank and sending unit is good insurance against having rust in the carb.