There aren't any larger off-the-shelf rotors that fit directly keeping the 4.25" pattern. People are generally changing to 4.5", or simply drilling the 11.65" Cobra rotors with the 4.25" pattern.
The Mark VIII rear calipers are the same as 93 - 97 T-Bird and Cougar. They are designed for the thinner rear rotors used on those cars, compared to 89 - 92. The Cobra rotors are the thinner style as well. So if your car needs to have the rear calipers replaced anyway, and you are going with Cobra rotors, then changing to the 93 - 97 calipers, anchors, and pads makes sense.
I do currently have some brackets ready to go. Send me a p.m. or e-mail if you need to order a set. Still $85 shipped, including hardware.
thx,
Ed N.