What's needed to upgrade?

Where do I get said adapters?

Dr. Diff sells them for $90

http://www.doctordiff.com/disc-brake-slider-caliper-mounts-backets-for-11-3-4-rotor-upgrade.html

Will the prop valve from the 79 aspen work?

Probably. I wouldn't be surprised if they were the same. Depending on the rear brakes, the Aspen may have had larger diameter rear wheel cylinders. That could alter the bias a little. But that's a pretty well known trick for fine tuning brake bias anyway, changing to the later wheel cylinders to keep the rears from locking up early.