It's not the fitment issue. I've seen plenty of Wilwood kits for the rear of the SN95. The issue is that 8.8 is a C-clip rear end. What this means is that axles are held in place by c-clips, and have a little side to side play. This play will move the pads, creating something called pad kickback. If you ever experienced it, you would know that first good step on the brakes would be hollow all the way to the floor. giving you no stopping power until you have some pressure in lines again, kinda like when you just compressed pistons for pad change.
What can you do about it?
1. Run with stock rear brakes. Upgrade to Cobra larger rotor kit (it's super cheap), get an aggressive pad to match the front kit, and you'll be all set. The whole reason why Ford used the stupid screw in caliper, is to directly deal with the downside of the c-clip rear end.
2. Get a c-clip eliminator kit. These kits convert your rear end to a 9 inch style axle tube, where the axle shafts are held in place by bearings at the ends of the axle housing. No play at all. Those kits usually run upwards to 500 dollars, for which you get all new brackets, new axle shafts with bearing, and and a few more things to make it all possibly. I've been contemplating on doing this myself, since one of my axle shafts is bent slightly.