I was looking for a larger tank to swap in years ago and settled on a 33 gallon tank from a diesel van. I think it was from a 1995 Chevy G30 with the 6.5. It has a 1 5/8 inlet and the vent tube is on the pickup assembly. I put it under the bed in place of the spare and it fit quite well.
I used my factory 1974 filler neck and used the round plastic filler assembly from a Jeep TJ and moved it behind the wheels. It has been working great for almost 10 years now.
There is also a 22 gallon version.
1974 Jeep J20, gladiator grill, 6BTA JWAC cummins, HX55, 4in exhaust to 5in stack, nv4500/ford np205, 14b kingpin axle, 14b rear axle, 4.56 gears, 37x12.50r16.5 Military tires.
G56 6 speed with dual disk clutch
Vintage air AC and Heat underdash combo unit
40x13.50r17 tires on 17x10 wheels