Wheel Size for 1968 Barracuda

I think someone tried to tell you that the sbp 15x6 you have are not oem, cause Mopar never made a 15x6 sbp wheel. (My oem 15x6 lbp are 4"bs). So, we don't know the backspace of the wheel that you have for the front, and it's important.
Frankly, I would mount 215/60x15s on your sixes, and 255s on 15x7s for the rear. A 15x7 with 4+ backspace will be easier to fit than a 15x8 or 8 1/2.