Could relays be used to supply power to the resistor block? One relay for each speed, and the fan still draws directly from the resistors. Fan switch controls relays only instead of full motor amps.
I worked on that car right before you bought it. Young friend found it somewhere and asked for my help to get it running and safe to drive. Small world sometimes.....
Check similar year Daytona (Pacifica and Shelby Z). Rear discs were offered and Shelby usually repurposed existing parts if possible. Bad news is rockauto did not even have listings for rear calipers, although they did show some rebuild kits.
Beat up the innernet and see if you can find a local...
Any one here know if the bolt holes in the head are blind or open to the water jacket? I've got one with coolant leaking seemingly from the deck/head, but the lower part of the manifold flange is stained along with one of the bolt heads. I rather not try to loosen anything without knowing if...