The problem here is that 8.25" rear axles aren't as wide as A-body 8 3/4's. Drum to drum A-body 8 3/4's are 57-1/8" wide with SBP axles and about 57-13/16" with BBP axles. A-body 8.25's are only 56-3/4". So with an A-body 8 3/4 and BBP axles you can usually run close to a 5" backspace, and even with SBP axles 4.5" of backspace isn't usually an issue. I'm a little surprised the 4.5" backspace on a 15x8 with 275's is so far off on this one, I did a test fit on my Duster when it had the BBP 7.25 (same width as an 8.25") and managed to get a set of 17x9's on there with 275/40/17's. It took a 4.75" backspace to get them off the springs, and realistically I would have needed a little more space than that to run it on the street. But those numbers are a little misleading because there isn't nearly as much sidewall bulge on that combination as on a set of 275's on a 15x8" rim.
But the measurement says it all, 5-9/16" is not a lot of room. 275's are 11" wide, so, they hang over a set of 15x8's by 1" per side. Throw in a 1/2" spring clearance and do some math, you get a backspace of 4-1/16" to fit those 275/60/15's on 15x8's on that particular car. That's a pretty narrow axle.
