The only 5spd autos are of relatively recent design and they would very likely have much less parasitic loss than a 4spd design based off a 904 or 727 that was designed in the 60's when fuel mileage was not much of a concern.
But, the late model trannies are all computer controlled and making one them work in a non computer car is not going a small task. Not mention the surgery to the tunnel to make it fit.
Best bet would be the 4spd A500/42RH or A518/46RH. These have electrically activated lock up torque convertors and OD gears but its pretty easy to make them work. However, either of these trannies is going to require significant tunnel surgery to make room for the back end of the tranny that contains the OD gears.
If you don't want to cut up the car and your pockets are deep Keisler is selling new GM 4L60 trannies with a mopar bellhousing that will fit.
