Because your truck is a 3/4 ton (3600) the options for under bed fuel tanks are limited, or at least that was my experience. Most of the tank relation experiences you may read about are 3100s and the frame rail spacing is substantially different from your truck.
I moved my tank out of the cab for a variety of reasons including changing to more modern seats (although I don't know that the original tank would not have fit behind them).
I used a 16 gallon tank from Tanks Inc. located behind the rear axle, between the frame rails, with the filler routed out the passenger side under the bed (through the bed side) with the filler cap located on the rear portion of the rear fender. The tank had a number of options including a fuel outlet that I connected to an in-line electric pump (but could be connected to an engine driven fuel pump). I routed the tank vent line out the driver's side, under the bed, through the bed side and mounted the vent high inside the rear fender.
You can find information on my tank installation in
my project journal or feel free to PM me with any questions.
