Well, I was wrong, the factory 7" wheel has 4" backspace at the bead and 4 1/2" at the lip. I took a few pics to show where I measured from.
First is at the bead...4"
Second is the overall setback of the tire... almost 5 3/4" This is the distance you need to know for clearance anyway. Having a narrower tire will provide more backspace. I couldn't get the camara down level on these pics to accurately show the measurement.
Third, you can see how much sidewall bulge is present with the 7" wheel. This will straighten up quite a bit with an 8" wheel.
I put this on the car to check for fender and frame clearance. Tire to frame is 1 1/2"
Tire to inner wheel well is close...1/2"
The 275x60x15's I have are on another car, a 1966 Chevelle and they are close but don't rub. It is 30 miles away in storage for the winter. Adding another inch to the wheel width and going with the 5" backspace that fiesta62 has should be good. If in doubt, consider a 255x60 tire but be aware that it is almost 2" shorter than a 275x60