Go find a civvy pickup at a salvage yard with the right tank, pull all the bracketry off the frame with it. Bolt it in the existing holes in your frame.
Plumbing it is a little more difficult. If you can get all the lines and the tank switch when you get the tank, do that, otherwise, you are going to have to locate a tank control valve and run the hard lines.
For the switch in the cab, you can either cut your dash to accommodate the factory tank switch or get a regular toggle switch for under the dash, then run the wiring for the control valve and switch (either grab from the donor truck or make yourself).
Finally, you'll have to either cut your bedside and have the filler neck grafted in, remove the entire bedside and replace it with one with a fuel door, or replace the bed with a dual tank bed.
Later,
Joe