I'm fairly new to auto repair, so my apologies if I'm off with the terminology. I've been helping a friend replace the shocks and springs on his 1985 W126 300SD. We followed the procedure in the wiki PeachPartsWiki: Replacing the Rear Shocks & Springs.
It went well, although definitely need to replace the sub frame bushings, but for now we just need to get the car moveable. While trying to reinstall the sub frame bushing bolt I can't seem to get it to thread. It seems as if its bottoming out, but it doesnt seem to thread and just slides back out. Is it possible that the threads are gone? Any advice would be much appreciated.

Look at that threads on the bolt and see if they are messed up.

See if you can get a strong flashlight and look up into the threads and see if they look buggerd up.

In the past people have had trouble getting the bushing pressed up into the Chassie. Perhaps that is the issue.

Site that has the service manual:
https://www.startekinfo.com/StarTek/outside/12253/?requestedDocId=12253

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84 300D, 82 Volvo 244Gl Diesel

You might have to remove the new subframe bushing and try the bolt to see if it threads in OK.

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84 300D, 82 Volvo 244Gl Diesel