17 inch Mustang rims..

Typically, mustang rims from that era are 17x8 with a 5.72" backspace.

If that's the size of your rims, and you're running the stock width rear and springs, you'll need about a 1" spacer in the back to clear everything. Most of those adapters are pretty close to 1", so that may be the ticket. You'll also probably need a small spacer (1/4" to 3/8") in the front to clear the tie rod end. Some people seem to be able to use the 5.72" backspace on 17's without the spacer, but others need a small spacer to make it clear.

Also, you'll have to open up the center register of the rims, turn down the outer edge of the brake hub, or use a spacer with a smaller outer register to use the Mustang rims.