Mechanically, the J20 parts are completely different. There is no connection in the similarity of the numbering. Your truck has some interchange with a '71 or '73. The J20 was first introduced in 1974, and there are many mechanical differences.
The body parts are basically the same from beginning to end, except for the beds. There are watershed years. 1971 was a watershed year. 1974 was a watershed year. Mechanical parts change a lot across watershed years.
There is a factory parts book that covers your truck here:
https://oljeep.com/edge_parts_man.html Look up the part you need, and find the Jeep part number. Then try and cross it on one of the parts sites like RockAuto to the part you need.
Don't assume the online listings are correct. Use the factory part number. Ask here if you are in doubt.
Jeep is an oddball manufacturer, and production of J-trucks was low. Ford sells something like a million light trucks a year, while Jeep probably sold 1/50th of that in a good year. You may have to work to find replacement parts other than regular service parts. Most of the parts for these trucks are NLA (no longer available) new on the aftermarket.
MTS sells a plastic replacement gas tank for your truck. They also sell a sending unit.
http://www.mtscompany.com/J-truck.htm Likely BJ's is selling the MTS products.
https://www.bjsoffroad.com/
I suggest you also get the TSM (factory repair manual) for 1972. This will tell you more about your truck than we could cover here in weeks. Available on CD-ROM at RockAuto -
https://www.rockauto.com/en/catalog/jee ... nual,10335 (NB this is a VERY reasonable price for these manuals. They are available other places, if you want to pay more.)
Welcome from Boston! We like pics.