Just wanted to point out another (cheaper, easier) alternative -- adapters. I'm running a set I picked up on eBay which convert the 4-lug to the 5-lug modern pattern (5x114.3). They're almost 2" wide which makes them perfect to use the modern Mustang wheels (the offset / backspacing then works out almost perfectly).
Using these I bolted on a set of 17" rims from a '95 GT to my '78 II. The rears fit just perfectly, the fronts I'll probably need to flare the fenders a bit. But both front & rear nothing inside interferes at all.
My turning clearance needs work but this is because of the overall diameter of my tire (it's almost 2" bigger than the stock tire). I'm running P245-45-ZR17's on my rims. Rubbing a little on the rear part of the front fenders. I expected this though, if I change over to P215-40R17's I'll be likely to fit just right. The spacers will get your backspacing with an modern wheel roughly correct, the rest is going to be up to you (as it would be with a 5-lug conversion).
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