87-90 had two physical tank sizes, a metal 14.5 gallon and a plastic 20 gallon. The metal tank used a circular mount sending unit that I believe secured itself with a lock ring. The 20 gallon plastic tank had a rectangle mount with 8 small bolts. In the 87-90 years, I believe there were two versions, one that was just a tube for the engine mounted mechanical fuel pumps (carb'd 4.2s) and one with a low pressure electric fuel pump for the TBI 2.5.
91-95 all use the same plastic tank, and many of them were limited to 15 gallon capacity with the vent tube. They made the vent tube extend further into the tank which vapor locks earlier to shut the gas station pump off. So even if you are only able to fill 13-14 gallons currently, your 91 should have the plastic tank capable of 20 gallons, you just need to do the vent tube mod to allow it to fill higher.
I think your best bet for a simple install is to buy the MTS Sending unit and bolt it right onto your tank. Maybe compare total length with your existing unit that you remove but I think it would work fine. I ordered from gas tank depot for the best price but you may want to order elsewhere for a better return policy (amazon maybe). I went all MTS (tank, skid plate, sending unit) knowing that all their parts would work together. My old tank also had a warped bowl at the bottom so I kind of wanted to get away from that with a new tank anyways.
