1990 R3500 454 crew cab dually need help

[wyattglock]

I had a similar problem with my 89 R3500 454. I put new AC Delco pump motors into the sending units. I believe I used the 486 or 386, can't remember which exactly after 12 years.
I also put a Ford solenoid on the fender well, and ran new wire to the starter. With headers, I had a really bad heat soak problem that would cause very similar issues to yours.
After all that, the tank selector switch went bad. If you replace it, make sure you get the switch that stays on.