904 Tail Shaft Seal Selection?

If it is a 1971 904 then it's any of these numbers.
Just depends on what brand seal you want.
Chrysler #2400163
Timken # 8935s
National # 8935s
SKF #15535
The PK number on the left side of the transmission would tell if it is a 1971.
These seals can be found from many vendors including eBay and Rockauto.