Well then you only have four viable choices;
1) the Mopar specific A500/A518
A-500 is the 904 based OD trans and requires the least cutting/modification.
The 518 is 727 based and big requiring some good work.
2) the GM 4speed,
A little pricey with the adapter plate
3)the GVOD,
Very pricey
and 4) no overdrive at all.
So not helpful
And below is so not helpful. Here, rethinking the combo is dumb and IMO states your dumb.
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.
One day AJ will realize something .....If you are worried about performance, spend your overdrive money, on getting the cylinder pressure up near 200psi, and you will spin any street-tires you can fit into the stock tubs, up to 60 mph.
190psi will take small closed-chamber aluminum heads,hi-compression pistons and, and a torquey fast rate cam. In a a Magnum, I highly recommend a Solid lifter, short to medium period, narrow LSA cam, and a matching TC. This will make a very fun streeter. Maybe not fast in the quarter. Maybe not quick. But can't-stop-grinning first gear blasts to 50/55, absolutely yes.That engine, on the street, will not care about rear gears. Well as long as 2.94s or better