slant 6 / 318 swap

The 8-1/4 rear can use a disc set up from a Dakota. The 8-1/4 rear doe an "A" body can be had cheap. Look here on the A body forum in the classifieds.

A note on the power differance on the auto tranny.
The 727 uses more power but not much. When drag racing these two side by side, it is a know factor that the 904 is quicker by a tenth or two. This is important for racing but not so important in the street. The 904 weighs less, uses less power and is more compact. It is plenty strong and can be made very strong should you ever decide to go drag racing with it.
It will stand with the 727.

But as mentioned, the 727 size can be an issue in a big way.

The newer A body cars are just easy to come by. There larger engine compartment lends itself to engine swaps a bit easier. Any engine can fit in. Parts are also easier to find due to the popularity.