Desperado,
Are you trying to install a rearmounted tank where the spare tire is located? If so a Suburban or Blazer tank will fit. Get a free catalogue from
www.lmctruck.com. Along with some from
www.yearone.com and
www.cheyennepickup.com. These catalogues have great details on suburban and blazer tank parts and location. You can find all that you need in the pics. Kinda pricey but you can hit the boneyards for what you need and buy what you can find.
If you want to install a tank in the bed over the spare tire then you have see if one of the guys here will reply. I've seen this type of mounting before. They use electric fuel pumps and the tanks bolt right the bed floor.