Skinny fronts with MP disc conversion??
I noticed this too Jamie when I bought my MP kit. After looking at the parts (not installed yet) I don't see why you couldn't use a 3 1/2 " wheel. It's just stock BBP brakes like used on 72-76 era Darts and Dusters. I'm sure there are tons of racers running this wheel.
I plan on running 7" wheels on the front anyway, so it doesn't effect me.