The low-buck way to go is to clean the tank. Old fuel will form gunk in the tank that is like the "diarrhea out the back end" you reported. I've been successful using an old motorcycle technique. The A-body tank isn't that much larger than some bike's.
I've found that this removes rust as well as gunk in tank. I did this with the both of my current cars. In the case of the Demon, the process exposed a hole in the tank that had been covered by gunk and undercoating.
- Drain the tank, remove sender unit/pick up tube, drop tank.
- Use a derelict sender unit to seal the sender unit hole.
- Add about a quart or two of solvent and a pound of gravel to the tank through the fuel fill hole. Gravel should be rough edged and about the size of a jawbreaker candy.
- Cover fuel inlet hole with hand and shake tank violently for 45 - 60 seconds. Vary angle of shaking periodically to expose more of the interior to scouring effect.
- Pour gunk out of tank into a large basin.
- Carefully inspect tank for results. If there is more gunk in tank, repeat with fresh gravel and solvent.
2 Safety Warnings:
- Dispose of waste gunk/gravel mix properly. Somebody's pet may get into it or your kids may think the slime is cool and play with it.
- DO NOT use an open flame or strike an arc with a welder anywhere near the open gas tank. It is a vapor, not a liquid, that explodes in the engine.