Depends on engine, which wasn't exactly specified. The 216-235-261 engines use either a two piece rope seal or a two piece neoprene seal. Either can be replaced without removing crankshaft. Just remove oil pan and rear main bearing cap to access. Changing the rope seals used in '55-earlier 235s requires a special tool, available at NAPA.
Now V8s are a different story. Most SBCs used either a one or two piece seal. May have to pull crank or tranny to change the one piece seal in a small block V8. I don't know for sure because when somebody says "rear main" and V8 in the same sentance, I try to blend in with the wallpaper until I can make my escape.
good luck
chip
