IMO you'd be better off getting a longer driveshaft.
To convert a short tail th350 to a 9" tail you first have to locate the parts, you need a tail housing and output shaft.
You can do the swap without taking the pan off but the pump and all the internals have to come out of the front of the trans because the output shaft is like the third part that goes into the case.
Then you have to ask yourself if you want to freshen it up while you've got it apart.
Probably cheaper and easier to get a different driveshaft.