245/60/15's on 15X8 rims

6 1/2" to the springs from the drum face? If that's right, those aren't stock location springs. That has to be a 1/2" offset. That assumes you took the measurement with the tape perpendicular to the mounting face and parallel to the springs- ie, you didn't measure at an angle. Do you have any pictures of the spring hangers? Or of the wheelhouse, springs, axle etc? Something doesn't add up there, the measurement to the springs should be more like 6" for the stock location and BBP axles and brakes.

Anyway, let's assume the measurement is correct. Working with the side that has less room, that means you have 11.25" to work with. Assuming a 1/2" for clearance to the quarters and springs, that's 10.25", so yes, you have enough room for a 255/60/15. So then there's the backspace. You have 4.75" to the quarter, so, lets work backward from the front space. You need a 1/2" of clearance, 4.25". With a 15x7, you have ~1" of tire overhang past the rim, so, 3.25" is the front space. The 15x7" is 8" wide outside to outside, so, the minimum backspace needs to be 4.75". And really, I assumed the 255/60/15 would be 10" wide on the 15x7, so, if anything you'd want more like 4 7/8" (4.875"). On a 15x8 that would mean 5.375" backspace.

You have a ton of space to the springs according to your measurement, and not that much to the quarters, which is why the backspace numbers are high. The measurements to the quarters make sense to me, 5" is exactly what I have and what I've seen before, the 4.75" side is tight but again not unheard of because that's just bodywork. But the measurements to the springs read like a 1/2" offset, because the drum to spring measurement is usually pretty accurate. If one side was an 1/8" off it would be one thing, but for both sides to be a 1/2" off is a big difference.