I figured its worth a note, any time B.S. is mentioned for the X.. 5.25" is found 99% of the time. I finally took a ruler to the 7" rims, and came up with 5.5" .. I did some number crunching, and it does indeed work out to 5.5".
Example
Known offset of stock rims is +40, listed in the manual,
AR 767 7" rims is listed as 0 Offset/3.95 B.S.
40(difference in offset) divided by 25.4 conversion = 1.57"
3.95 + 1.57 = 5.52 Backspace of stock rims.
Not anything important.. but something when comparing to 8" rims.