Does anyone know if a big brake kit for the 05-07 mustangs sticks out further than the stocks in the rear? Because I plan on upgrading the brakes in the future but my rear wheels just barely clear the stock brakes in the rear, and that's with a 3mm spacer.
You really should look into this further. I bought a Saleen front brake kit for my 07 GT and with the 20's I'm running the kit wouldnt clear the rotors and I was screwed. Call some people and find out before you purchase a brake kit. If your clearance is that close already I doubt a 14" brake kit will fit. But I'm no expert.
Most aftermarket brake suppliers have a downloadable PDF file on their website which you can print out, paste onto card and try against your wheels. As with the above poster, I bought wheels that nothing would fit so I just bought slotted rotors and Hawk Performance pads which improve the braking anyway.
The Mustang has pistons on the inside of the caliper. A lot of aftermarket kits also have a piston on the outside of the caliper so require a lot of additional clearance so even with 20s, you're screwed.