A body BBP axle length for 8 3/4"

In all reality the distance from the inside of the bearing to the end of the spline is the measurement you need.

Regardless of cut down and resplined BBP axles or redrilled SBP axles the only diference will be that the BBP axle will need B, C, or E body backing plates and drums and the SBP will need SBP backing plates and drums.

I assume (and we know what happens when we assume) that 8 1/4 brake backing plates and drums are the same as B, C, and E body parts.

The overall distance from flange to flange will change between SBP and BBP axles due to the diference in the wheel flange to bearing spacing.