rear disc brakes on 8 3/4

The ford caliper can be used with 2 sized rotors depending on the wheel used. For 15s a modified Toyota rotor is used (10" diameter). For 17s an 11.75" rotor is used - straight from a 90s Cobra. The pads are available anywhere.

what Kind of Toyota? model or part number? and how modified?

Ford Cobra is a simple new Mustang?

it is exactly this one?

http://www.doctordiff.com/rear-disc-brake-kit-10.7-rotor.html