A few years ago, one of my friends purchased a 4 Cylinder fox and did a GT swap into it AFTER adding subframe connectors. His poor GT chassis was twisted beyond recognition from all the abuse that was done to it over the years. Then again, I got my @$$ handed to me by a late 80s Camaro with a 383 stroker under the hood.
If I were going retro, I'd get a notchback then modify it. Even back in the day, it didn't take much to make a 5.0 notch smoke the rustang GTs from the same era. In fact, the old dragster secret to making them go faster was to drop a Chevy small block under the hood. That way you could pi$$ off both the die hard Chevy and Ford guys!