Classic/Antique Car Repair: 1971 mustang mach 1 transmission leaking, mustang mach 1, drive shaft

Question
I have a 71 mustang mach 1 with a 429 V8.  The rear seal in the transmission is leaking. I know when I bring it to the dealer the traditional method is to remove the tranny, take it apart, fix the seal, and then put it back on the car. Do you know if there is a cheaper and possibly easier way to have it fixed but still having the same effectiveness?
                              Thanks

Answer
If it is the seal at the drive shaft that is leaking that is a simple job. Drop the drive shaft and remove the old seal. Then with a seal driver tool install the new seal and replace the drive shaft. If it is the gasket between the extension housing and the transmission case that too can be done in the car without removing the transmission. In fact it is the factory recommended way to do the job.