The problem is the 5.5 BS using a 275/60 is already tight on the fender lip. So any movement outward would be an issue.

The 15x8 and 15x9 having the same back spacing means the 15x9 is 1 more inch outward. Now the 15x9 and a 6.5 BS would be one more inch inward and same BS on the face. Technically a 15x9 and a 6.0 BS would be ideal. That would be =.5 and -.5 and keep the tire in the same place with more meat.

I can always move a wheel outward using a shim or spacer. The opposite direction... good luck. And once you mount a tire on a wheel.. you bought the farm

Hell the fix is minitub the car so I can make the wheels fit. Thats the Moby Dick fix... come hell or high water.. i'm gonna get that whale(make it fit)