Try Stainless Steel Brakes. They make a rear caliper with parking brake (Force 10 Sport R1 Single-Piston Aluminum Caliper) that is a direct replacement for the '79 Trans Am rear calipers. It has the 2.125 piston with a nice aluminum housing. They are the calipers I will be using for the rear of the 442.

http://www.stainlesssteelbrakes.com/products/sportR1/

By the way, know of a good source for slotted or drilled '79 T/A rear rotors?