The stock A Body 8.75" brakes are 1.75" wide.. If you buy axles and try to use the drums from a later car with 2.5" wide brakes, the drum will hit the backing plate. The backing plate for the 2.5" brakes is "offset" or "cupped" more towards the center of the car to allow for the wider brake shoes to still be centered the same.
I find it easiest to grab a whole unit off of the back of each side of a donor car, and start from there. That way you have the backing plates, maybe drums that you can have turned, and all of the other hardware, some of which is no longer available new.