Dan,
partial answer:
You need a special pin socket for the flange nut, it's been discussed a lot so some searching should find info on it. It's also not too hard to make one if you're so inclined.
Parts: new lock ring for the nut, and I recommend replacing the flex disk and rear motor mount and bellows while you're at it. I don't have part numbers off hand, but the usual parts suppliers should have no problem with them. Failure to replace the flex disk may lead to driveshaft or output flange damage, so it's (not-so-)cheap insurance.
The book says you should loosen the exhaust system to get to the bolts that secure the cover plate under the transmission, but you may be able to get the bolts out with a wobbly extension and short socket.
Good luck!
George Davis
'69 280 SL Euro manual