On that transmission the tailshaft will not come off without disassembling the transmission. Once you have the two sections separated you can get to the snap ring if needed. You can replace the rear seal with the housing in place.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine