Help me confirm my measurements and back spacing thought

If you take 4 7/8" + 7" that gives you 11 7/8" total fender lip to spring spacing, at the most. Your 275 tires are about 11" wide at the sidewalls. Not including a margin of error, sidewall flex from cornering, side-to-side body tolerances, etc... So you have 7/8" total clearance, or 7/16" on each side at the most. Did you measure both left and right sides of the car? They are usually different. It doesn't matter what backspace wheel you put on it, as far as I'm concerned a 275 tire just isn't going to fit without some kind of body surgery.

{By the way, by my figuring, the theoretical backspacing for your car should be 5 9/16" (or 1 1/16" offset) with the 8" wheel. Same as what 72bluNblu came up with.}