Hey I can help. I am a tire technician at American Tire. Ok the '99 mustang gt has 17" 'Bullitt' wheels. They are 5-114mm bolt pattern. Now we have a problem. Your '86 Mustang has 4-4.25 bolt-pattern. They won't fit. I wanted to get the Bullitt rims for my '83 but dealt with the same problem. You can get a conversion kit to make it a 5 lug. They cost a few hundred bucks. One place you can get that from is Latemodel Restoration Supply.
www.50resto.com As for the wheels, you can even find them on eBay. Or grab a MuscleMustangs And Fast Fords Magazine. These wheels should not be hard to find. Good luck and just ask if you need anymore help.