I've got the Mustang II spindles on my car with 11" Granada rotors and some GM calipers. I have hated the setup since day one. (Way too much pedal preasure)
The rotors warped last season so it is finaly upgrade time.
I believe Wilwood has a kit for the 4 bolt wheels.
I liked Wilwood because they had so much info online as to exactly what would fit my car.
I kept searching around and came across
www.HotrodsUSA.com.
In my case they have exactly what I wanted. 11.75" Wilwood rotors with 4 piston calipers and no change to the wheel offset.
Watch out for these kits changing your wheel position. Many kits will move the wheel out 1/2" or more to make room for bigger calipers. The kit I found has a custom zero offset changing hub that is then mated to Wilwood parts. It is a 5 bolt patern so it won't work for you.
Wilwood and Summit had a good selection of kits and rotors.
If you find a 4 bolt 11" rotor you may want new calipers too. I spoke at length with the owner at Hot Rods USA. He realy seemed to know is stuff. He said the problem of high foot presure I was having was caused largely by the single piston GM caliper. Once you go to 2,4, or 6 pistions the caliper works with alot less effort. I hope he's right because I just ordered my kit 2 days ago.
Harvey