Can Small Bolt spindles run Big Bolt rotors?

So, a couple of things to note here

1- A-bodies never came with BBP front drums. So, if you actually have BBP front drums, you probably have spindles from a different car (B/E body)
2- Your '75 Duster has the more desirable large ball joint upper control arms. You do not want to switch those to the small ball joint UCA's your other set up uses
3- There's no need to switch the lower control arms, they're the same unless they have sway bar tabs

By far the easiest thing to do would be to buy a set of reproduction 73+ disk spindles and run factory 73+ disks. You swap out the spindles, get the BBP brakes, and everything still uses factory parts for a '75.

10.95" Front Disc Brake Kit (Stage 1)

Looks like the full kit is currently out of stock for some reason, although the disk knuckles show as being available. I'd call DoctorDiff on Monday and ask what the ETA was on the kit.