heres the spec of the 05 GT rims:
5-Lug 4.5" bolt circle
6.295" backspacing
17" x 8" wide
(the V6 mustang rims may differ, but you asked in the gt tech, so thats what i assume you want)
the 66 will need spacers to fit. check out the classic mustang forum, as i believe some there have done this already.... the classics were made for reverse offset more than the new ones. (think basically deep dish and reverse deep dish)