I believe you can save the spindles and just swap to the 10" backing plates, but not positive about that. I have a few sets of the 10" fronts, and spindles too if you needed them. Of course, a switch to the K-H discs would be a much better upgrade, but also much more $$. Let me know if you're interested in a 10" setup and I'll put a set together for you!!
To answer your question, no 8 3/4 ever came with anything less than 10" brakes. As a matter of fact, the smallest brakes you could get on any 8 3/4" is in fact the 10x1 3/4" that the A-body cars had. That is also the only size available for an A-body 8 3/4" as they were all SPB axles and all the same size brakes for every year they were available!! There are 2 different style drums however, the flanged "Canadian" drums and the non-flanged style. Hope all this helps, Geof