Mike, to replace the seal the main shaft has to come out of the case. Now, whether the entire clutch and gear set has to be broken down as well, I can't say but if you're that far along, it can't be too much more money just to do it all.
Yep...all that just to replace a $5 seal...
If the transmission is in the car, try the trans sealer suggested earlier. I put some in another 53 I had and it actually worked.
