I would tend to agree with crashone. The guide rod mount kit comes with a new "tabbed" nut.
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=23K00S4YN23W06EQVC&year=1983&make=MB&model=300-SD-002&category=L&part=Guide+Rod+Mount+Kit If you can get a wrench in there or maybe some of those needle nose vise-grips. One thing for sure, use lots of frequent doses of PB Blaster. Once you get it apart, is sure would be a good time to do some extensive cleaning and rust treatment as this is an area that takes a lot of stress.
If you DIY, use a come-along to hold the bearing carrier in place and allow you to align things to re-bolt the assembly.