Converting rear drum brakes from SBP to BBP
Assuming that you're buying a set of BBP axles that will work.
Also recall that 11" rear drums came in 11x2, 11x2.5, and 11x3" varieties, and you can't swap drums and backing plates with those either. Since the BBP axle offset was the same for all of them, the axle flange is in the same place for all of those and therefore the offset for the backing plates on each of the 11" drum set ups was different. Meaning, if you have 11x2" backing plates, you can't just buy 11x2.5" drums and shoes and run those. Somewhere I have those measurements to tell which is which.
***Edit***
Found the measurements.
To measure the backing plate offset you want to set the backing plate flat on a bench. You'll need to hang the bottom 1/3 of the backing plate off the edge of the bench so that the mounts for the e-brake cables don't keep the backing plate from sitting flat. It isn't much, but they do change the measurement. Then, measure from the bench to the top of the backing plate through the center hole (for the axle) in the backing plate.
If you get ~1.75", you have 11x2.5" plates. If you get ~1.5", you have 11x2" plates. Those I know for sure, as I've measured them. I don't have a set of 11x3" backing plates, but the math would say you'd get ~2" measuring them as I described above. This is because half of the additional width is on the inside of the axle flange and the other half is on the outside to keep the axle flange in the same spot, as there was only one axle offset for BBP 8 3/4 axles.
Also, if you have a set of 10" backing plates without any hardware, shoes etc you can measure those to determine if they're BBP 10" backing plates or SBP 10" backing plates. BBP plates will measure 1.75", they're 10x2.5" and have the same axle flange offset as the 11x2.5" brakes so you get the same measurement. The SBP 10" backing plates will measure roughly 1.25"
Assuming that you're buying a set of BBP axles that will work.
Also recall that 11" rear drums came in 11x2, 11x2.5, and 11x3" varieties, and you can't swap drums and backing plates with those either. Since the BBP axle offset was the same for all of them, the axle flange is in the same place for all of those and therefore the offset for the backing plates on each of the 11" drum set ups was different. Meaning, if you have 11x2" backing plates, you can't just buy 11x2.5" drums and shoes and run those. Somewhere I have those measurements to tell which is which.
***Edit***
Found the measurements.
To measure the backing plate offset you want to set the backing plate flat on a bench. You'll need to hang the bottom 1/3 of the backing plate off the edge of the bench so that the mounts for the e-brake cables don't keep the backing plate from sitting flat. It isn't much, but they do change the measurement. Then, measure from the bench to the top of the backing plate through the center hole (for the axle) in the backing plate.
If you get ~1.75", you have 11x2.5" plates. If you get ~1.5", you have 11x2" plates. Those I know for sure, as I've measured them. I don't have a set of 11x3" backing plates, but the math would say you'd get ~2" measuring them as I described above. This is because half of the additional width is on the inside of the axle flange and the other half is on the outside to keep the axle flange in the same spot, as there was only one axle offset for BBP 8 3/4 axles.
Also, if you have a set of 10" backing plates without any hardware, shoes etc you can measure those to determine if they're BBP 10" backing plates or SBP 10" backing plates. BBP plates will measure 1.75", they're 10x2.5" and have the same axle flange offset as the 11x2.5" brakes so you get the same measurement. The SBP 10" backing plates will measure roughly 1.25"