904 transmission rear seal

If the parts clerk looked up 72 340 car he may have found 727 trans instead of 904. Maybe have him look up 72 slant 6 and see what you get
If If If, Since we know this is a piece work drive line, If drive shaft balance and/or yoke insertion depth aren't proper, the tail shaft bushing may be the root cause of leak. A new seal alone wouldn't fix it.