I think MBZ stuck a V12 in there and there is plenty of room on the sides to play around and that's nice when doing any engine swap BUT it's too tight for getting at the upper Bell housing bolts easily as 6/2 says also getting at the drain ducting is close to impossible with engine and trans in place.
. go for the unit R&R if you have the space, it's almost always easier in the long run....and this may be a longer run than you think...sounds like a fun project but i'm glad you are doing it and not me.
Good luck!