The bolt circle is the same. Here are clearance issues I encountered when I tried to fit my '97 GT 17" to my maverick just to see if they fit. On the rear, the wheel was up against the leaf spring before it was fully seated on the wheel studs. In the front I was actually able to get the wheel on, but it was on drivable. It would have rubed severely in turns and the inside of the rim and the top arm rubed the inside of the rim.
I would expect similiar problems on an early mustang, especially a '68 or earlier. That said I did see a '69 on the road with what looked like '96 GT 17" wheels on it one day. Now of course that doesn't mean much because I have no idea of what kind of mods were done to that car.
The more narrow 16" GT wheels may have a chance of fitting. I think the 6cylinder's 15 inchers would have the greatest chance.