They make plastic (polyethylene) gas tanks for these trucks. I remember some made a big deal where they would be safer with a side impact as plastic doesn't spark after that news story about them. MTS makes some. There are probably others also.
http://www.mtscompany.com/chevy-gmc.htm
Here was part of an article from fourwheeler about them:
Worried about strength? This is where MTS Company's tanks excel. Its one-piece tanks are made of high-density cross-linking polyethylene.
This cross-linking process makes them 35 times stronger than non-cross-linked polyethylene. They won't crack, even at 150 degrees below zero, and the plastic offers a memory which enables them to bounce back after an impact. They weigh less than a comparable steel tank, they're UV-stabilized and the walls of MTS tanks are 1/4-inch thick, where most metal tanks are 1/8-inch thick.