On my 70 Duster I currently have a stock 8 3/4 with sbp (5 on 4) axles. I recently picked up a set of bbp (5 on 4.5) axles to swap out. Will I need to swap out the entire drum brake assembly, backing plates and everything else?
In general, yes. Which axles did you get?
While there are a few aftermarket offerings of what amount to redrilled 4" axles that require you to redrill your original drums, most are axles that require you to swap to B/E/C/FMJ/truck-van-Dakota w/5x4.5BC and some Jeep Cherokee drums.
The difference is in the flange offset- the distance that the axle flange protrudes from the housing end. Factory A body rears through '72 use a unique offset that was not used on any other Mopar offering, necessitating the use of their own style of backing plate and drum, which dimensionally do not interchange with any other Mopar. Most 4.5 BC conversion axles use the standard Mopar offset, and require the use of the backing plates and drums listed above.
All that being said, most suppliers will specify in their spec sheets which brakes their axles are compatible with.
