Oil leak in auto transmission.

Ref earlier post regarding spun cam bearing. Engine is shot and i have bought a used engine to replace it. The transmission is leaking and when the engine/transmission was removed there was a litte oil on the floor indicating that it is leaking also when engine is off. It seems that both oil seals for the drive shafts are leaking to some extendt but it is also leaking from the underside of the casing. To me it seems that either the oil pan or the cover over the differential housing is leaking. The transmission now sitting on the floor i figure it would be easy to fix this now. Does anybody know if i should use a gasket or is it ok to use only RTV sealant. Also will any parts fall out if i remove the cover on the differential housing. The right oil seal for the driveshaft seems to be easy to pry out with a screwdriver but what about the left one?