I have a buddy that said the same as Ryan. He said you can pull the axles out of a 4th gen rear
and they will swap into a 2nd gen rear
Cant verify this, but supposedly he has done so. Get the rear from a junk yard for cheap and the wear parts should be able to bought off the shelf.
Are you running over extended h/p? I have disc front and drum rear in my 77 t/a, 400 warmed over. Car stops just fine. I see no need to change what the factory engineers came up with. Most of the braking should be done in the front anyway.
I have a 79 with 4 wheel disc, will be curious to see how it handles in comparison.