Until the 1988 model year, the GM trucks all had saddle tanks. You can use a rear tank from a Suburban (28-30, 34, or 40-gallon, depending on year) or a Blazer (22, 28-gallon) tank if you also get the rear cross-member and straps. But, then the rear fuel tank becomes your low point for departure angles.

Or simply use any suitable tank you find that will fit the available space. However, you might check into a fuel cell for both your safety and public safety. Summit Performance Parts and similar businesses carry fuel cells.