8 3/4" narrowing - Dimension Help

Disregard the 7.25 rear lengths, do you no good.

You have an 68-70 B 8.75... good starting point.

You can use the axle as is, especially if buying new wheels, just need a bunch more offset. Want more dish, need to shorten and it's easier to get tires on/off the car with a shorter axle.

If you are putting the BBP axles in and want it the same overall length of a factory A body 8.75 (axle flange to flange), total length of the bare housing needs to be 52.075"
That should make the total width with brakes 57 1/8"

The difference in overall length at the 52 5/8 housing with BBP makes it right at 57.675, only a little over 1/4 further out on each side compared to the 57 1/8" factory SBP unit.

Axle tubes are the same length, there is NO OFFSET in the housing. The center section sits dead center and accounts for driveshaft offset. Chevy guys F this up all the time when shortening mopar 8.75 rears.