Hi tcodi, the Mustang would be 4 1/2 (ford used the 5" on the big full size boats, cop cars etc).

I know what you mean about finding out about back spacing. Here's one way I use to figure it out.

Mount up a fairly reasonable size tire that you can get into the wheel well. It works best if you can use the size of tire that you will actually going to be running, but it's not critical, and in some cases may not be doable if the rim your using isn't close to the right offset.

Once the tire and wheel are mounted on the car lower it to ride height. Measure the distance from the outer sidewall to the inner fender (if the inner fender tappers in at the top of the wheel travel, make sure you take this into account).

Now measure the distance from inner sidewall to the closest part that a tire can rub on. This is usually the inside inner fender well, but it could also to the rear leaf spring/lower control arm.

The difference between the 2 measurements is the amount of offset that either needs to be added/subtracted to the back spacing to center the wheel in the opening.

Having the wheel centered in the opening of course allows the largest tire to be used.

Hope this helps.