11" to 12" rotor swap

12" aftermarket, or the later model 11.75" mopar rotors?

If its 12" aftermarket, you'll need caliper brackets, calipers, rotors, and maybe also new brake lines and an adjustable proportioning valve to get the rear brake bias right. Some of those kits also recommend a new master cylinder. You'll also have to make sure you get the right kit for the type of spindle you have, some work on drum spindles, others on disk spindles.

For the later 11.75" mopar rotors you need '73-76 spindles, new rotors, and caliper brackets. The calipers are re-used, and the brake lines can be made to work if routed properly. At least on my Challenger the bias was actually improved since it was too far rear-biased to begin with, so you probably won't need an adjustable prop valve if you already have disk brakes. If you've already got 73-76 disk brake spindles, this is a piece of cake, and the braking improvement is very noticeable. Even on a big 'ol e-body. :D

And for either application you should probably replace your wheel bearings while you're at it unless you've already done this recently.