Chrysler may have made 100 different fuel tanks between ’65 - ’90. With that said, they might have made 700 different sending units – with only a few that might be interchangeable.
The FMJ body and early front wheel drive fuel sending unit (to tank) seal might be the smallest diameter they used (as well as the diameter of the fuel filler neck).
When the front wheel drive cars went to fuel injection, that diameter got a lot bigger.
The only fuel senders to fit FMJ tanks are, well, FMJ senders – and even then, they will only fit within the three different tank groups.
The three different fuel tank versions are: The ’76 to mid ’77. The mid ‘77 to ’79 (I think) and then ’80 to end of production. All three tanks required different senders – with variations of each.
It is not that hard to remove the 5/16” tube from an old sender and replace it with a 3/8”. Epoxy works well. Also using aluminum tubing makes the bending a breeze.
BudW