Greetings. If this has been asked before, please point me. I did a search but didn't see anything matching.

Bike is an old MTB North Country 2X6 with Shimano derailleurs.

Cable mount for rear derailleur appears to have been threaded through the derailleur mount(I got the bike very used). New cable housing is not threaded and I need some way to mount it for proper function. I could weld up a cable mount, but I'm wondering if there's an easier way.

Can you post a picture of your setup? What you are describing doesn't make sense to me.

I can once it's daylight. I'll see if I can get a shot with flash that's clear.

Not sure what you are describing. Can you post a picture?

It sounds like your cable housing has gotten stuck somehow to a barrel adjuster, or the barrel adjuster is missing and you are looking at the threaded hole where it goes. There is no such thing as threaded housing.

Barrel adjuster may be missing, then. Cable was snug at the hole but wouldn't stay in place. I feel extremely noobish--haven't worked on a bike since 1983.

Note barrel adjuster.

It looks pretty well stripped out. Are there larger ones sold, or should I try to weld a nut on, or just get a new derailleur?

You can't tell if the threads are stipped as long as the cable is there but if it is, than just get a new derailleur.

A new barrel adjuster with a lock nut would get you rolling again if the threads are questionable.

When you get a barrel adjuster the lock nut will serve as a stop,
against the edge of the hole, even if it is stripped out.
put a flatwasher on to span the hole, reduce its size.

You can buy a brand new Shimano MTB derailleur for 20 bucks or less. Why bother to fix that rusty thing?

New derailleur, then.

Eventually, the bearings will fail and the only original piece on this bike will be the frame.