I'm running the Bogart D-10's on a 86 Gt....
They custom build these wheels to your needs. They are pricey however, you can buy a complete set of Weld wheels for what I paid for just the rear set. I decided to go with the 6" backspacing so I wouldn't have to do any fender/inner fender beating. This all worked out really well. The 275/50-15 fit great now. Havent tried the 275/60-15 yet, but I don't think I will have problems with them either.
Good luck, Teddy