Transmission Output Shaft Question

CudaFactHackJob said

I think 65, 66, and 71 or was it 68 on the rear seal? I never go by the year cuz they have all been apart before and parts changed.

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So say, for example, i want to change the tail on my 85 A999 for small block with one from my 71 A904 slant 6, i wood need to change the seal based on the year of tailshaft used?