Here's what I found, a LONG time ago on my 83.
Tank is steel. Straps are steel with "supposedly" a rubber liner on the straps. Rubber liner was MIA in many spots. Straps chafed pinholes in the tank, which grew.
If that is what you find, do as I did (cheap). Drive one side of the truck up on ramps to pool the fuel away from the leak. Buy fuel tank sealant (it should be a two part paste you have to mix) and apply it pretty thin, several coats, let dry in between. I basically glued and encased my straps with that crap; no leaks. Obviously, you have to loosen/remove one strap at a time to do this correctly and the surface must be dry.
If you can't find the tank sealer, go on line and google "MIL-S-8802". That's about the best you can get. Note that it comes in several cure rates, anywhere from 1/2 hour tacky, to 24 hour. Your preference. There's also a product called Pro-Seal = same stuff.
^^^^ Even better! Hard to beat $54!