17x9 wheels are not advisable on a 65-66 Mustang. Even with tthe correct offset, you have very little space in the front wheelwell for proper tires. A 17x8 is doable, but tire selection is critical and depends on how much lowering you may have done and what other suspension mods are in play. It's tight.
You may also need to make some mods to use an exptremly low profile tire as well. The camber curve on the early Mustang is not very friendly to low profile tires like that. Without getting into the specifics of suspension science, the early cars have a tendency to "roll positive" , riding the edge of a low profile tire in corners. This leads to poor traction and extreme tire wear. The cure is to alter the geometry by lowering the pivot point of the upper arm, but that in itself leads to fitment issues with wide tires.