I have zero experience with that one beyond a few hours of gazing at an MG and an Olds 215 sitting in a garage next to each other, but, just to throw out a few casual thoughts....
I'd bet the pinch point is going to be at the upper-a-arm inner mounts. Narrowing a Mustang II front would probably mean shortening the lower a-arms to keep a inner pivot dimension to work with the steering rack and then as the uppers are already pretty short, you'd be moving them inward as-is, but then those and the cylinder heads would be trying to occupy the same space.
I'd place my bet on fabbing from scratch starting with the narrowest rack you can find, and using McPherson struts. Upper mounts for that would get weird but I don't think anything else would fit.