Dodge rear swap

If the new rear end has the same gearing as the old, you won't affect performance any. One thing to watch when swapping rear drums for disc brakes is the line pressure. To do it professionally you would want to swap to a newer master cylinder designed for 4 wheel disc brakes (with new lines running back to each rear wheel). Or you can use an aftermarket proportioning valve (check out Summit) that gets installed in the line after the factory one (the factory setup is a single line running to a t block then to each rear wheel). It takes a little tweaking to get the right line pressure for the rear discs.