Find a wrecked '87-92 5.0 as a donor. Park the two next to each other in a garage and switch EVERYTHING mechanical over. You can go with the EFI if you're EFI savvy and have the time/patience, or just stick with carb for simplicity. Switch the rear end/axle, the front/rear
suspension, transmission, everything. The thing about Fox Mustangs is that they consistently got better over the years 79-86, but remained almost mechanically identical between 87-92. I say don't get a 93 motor because the pistons are hypereutectic and don't handle power adders as well as the forged units in the earlier cars.
Aside from the car's insides, keep the '79 body intact. You don't see a lot of straight Foxes that old.
Lastly, lets see some pictures!