it really depends on your method of replacing the chain, most technicians including myself replace the upper guide rails and then roll in the new chain by connecting the new one to it and turning the engine by hand. i would say this will be easier done in the car, but would also be easy with the engine out and attached to am engine stand.

if you are going to remove the front engine cover and replace it like that with all the rails, it would be easier with the engine out and also mounted to an engine stand. this method is the most invasive and is quite involved

but to replace the front seal on a transmission the engine does not have to come out, i have done many by simply pulling the transmission, if you go to the extent of pulling the trans you should also at minimum also replace the o-ring that goes around the front pump, this has been known to leak more often than the actual radial seal.

__________________
1967 250 S
1967 280SE
1968 250S
1971 280 SE
1973 220
1974 280 S
1975 450 SEL
1976 220 D
1979 300SD
1983 300 SD
1985 230 TE euro
1986 280SL
1986 500 SEL euro
1986 190E 2.3
1990 300 SE
2003 C230 K
2004 C230 K
1987 560SEL
1987 300SDL now in canada
2005 C230
1998 C43 AMG Sold to DON
1993 190E 2.6
300TD Former daily driver Totaled
current:
R320 CDI
Ram 2500 Cummins