I would recommend same size tires. Why...
The Mustang understeers (plows) to begin with. Smaller tires, overpowered by bigger rear tires, only makes this situation worse.
As for wheels, the ROH 16x8 wheels were my first set. I ran 245-50's and then 245-45's on all four. The 45's were a soft compound and fit better (lower profile) in the 9:00-o-clock position of the driver's wheelwell on a 1989 fender.
My current combo is 275-40 R1's on 17x9-1/2" wheels on all four corners. Yeah, this took some work, but the results are well worth it!
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Darius Rudis
Roadracer: Home built t-arm/panhard suspension powered by an S-trimmed motor :-)
My Mustang Page