Considering a lot of folks run 15x8 with 4.5" backspacing with stock wheel tubs and offset springs, if you wanted to widen the tubs to the frame and run a 15x10 wheel, the extra 2" of wheel width would be on the inside giving you 6.5" backspace on a 15x10 wheel. Depending on how high or low your car sits, that amount of backspacing may make taking the wheels off a total pain. You really do need to mount up the rear and take some measurements though as every car is different. There are just way too many variables for you know with 100% certainty without getting your head under there and taking measurements. What brakes are your running? Even the difference in brake offset/standoff between SBB and BBP could throw things off.