Depending on wheel hub thickness, you may not be able to go over .25 spacer with the stock length wheel studs. Yes, the fixed, bolt down versions are the best, but I will admit to having heaped piles of abuse on .25 spacers with varying results. Of course, the quality of the spacers is also a point of concern with the cast aluminum universal design being bottom of the barrel and machined billet hub centric ones being best.
Most kits to adapt larger disc will also move the wheel spacing out as well. I know for sure the AR Engineering kit for 13" rotors is designed specifically to mount Mustang wheels in the correct spot. Wilwood also makes mention to check backspacing as it changes using their hub kits.