I don't believe they made 9" brakes for the 8 3/4's, just the 10's. And A-body 10" rear drums were different than everything else, they were 10x1.75". Everything else came with 10x2.5" rear drums, or the "big" rear drums which were 11x2.5".
I also don't think the 9" drums will swap over to the 8 3/4 for several reasons. But, even if you could figure it out I believe you'd need new axles anyway, since the length of the A-body 8 3/4 axles is different, the backing plates used a different offset (also true of trying to put 10x2.5" or 11x2.5" drums on an A-body 8 3/4, need new axles).
Either get an adjustable proportioning valve, or ditch the 9" fronts. They were sub-par even back then. If you're trying to stay SBP, I'm sure you could find someone with a set of 10" front drums they'd be willing to get rid of, lots of folks swap to disks and drop even the 10" drums.
