well, it took me longer than i thought to dig out the spindles to compare, but take a look. the "new" spindle is the common aspen,volare,dart, b body , etc. spindle used for drum to disc conversions. the "old" spindle is from a mid 30's plymouth. 37, i think. 38 should be the same, and i believe 33 would be very similar in regards to length and bearing surfaces. the outer bearing surface is .749 diameter on both spindles with the length of threads being .937 on both, and the threads are identicle at 3/4x16. the bearing seat length on both is .678 long. the key slot is .187 on both, with both being the same length.
the inner bearing od. is 1.249 on the "old" vs 1.3745 for the "new". the bearing seat length is .958 from the seal surface to where the "taper" begins on both.
the seal surface od. is 1.7375 x .375-.400 long[approx] on the old vs 1.943 x .570[approx] on the "new".
the length of the spindles from the front to the seat of the inner bearing is 4.767 for the "old" vs 4.700 for the "new. it appears the length difference is in the taper area, and should be , for all practicle purposes, not of any concern.
however, the chamfer at the inner bearing seat is different between the two spindles. if a "spacer" were to be machined to use the factory inner bearing of the 10 3/4 or 11 3/4 rotor, you would have to take that into consideration for a proper fitment. you could also incorporate an inner seal adapter with the bearing adapter. this would be a somewhat simple item to fab. i haven't had time to research a conversion inner bearing or seal[ if such exists], but i will try to do so soon. hope this info helps.