Yes you can do it in car but not easily , if it has the lever style side cover it is much harder to do that the earlier ball detent type of cover used before 70 , it is possible to tell the difference before removing it , most 70 & newer use the lever type cover & the cover has to be installed with the levers in the cover , the earlier detent type the levers can be in the trans first .
The side cover uses shoulder bolts in 8 - 10 holes but in the 3 o clock position it uses a double deep shoulder bolt , this aligns the cover properly with the case this can Only go in the one position , the last 2 bolts are longer where the cover is raised around the reverse area .
As above make sure the leak is not elsewhere , it is unusual for the side cover to leak unless the gasket was damaged when it was installed . let us know if you need help .