Well then you only have four viable choices;1) the Mopar specific A500/A518
2) the GM 4speed, 3)the GVOD, and 4) no overdrive at all. The first three will require a lot of surgery to make fit, and perhaps a bit more effort to make work.
#4) just requires a rethink on the engine combo. I well thought out 5.9 in a streeter,should not need 3.73s in an early A, with an automatic. Change the engine build spec and run 3.55s, or even 3.23s.
Check this;
3.73s with the standard 2.45 low is a starter gear of 9.14.. And 9.14 divided by the 2.74low, A999 ratio, is 3.335.. Bottom line is the engine would not know the difference.