[68GMCCustom]

if the cab gas tank isn't worth saving you could drain it, clean it out, and cut the filler neck off and make a bracket to hold it in place. Then put the cap on and leave it.

Or....fill it with a piece of metal and weld it in, use some filler, and a little primer and your done. You can buy the precut metal piece at various dealers selling parts for these trucks...or make your own.