The lock-nut came in yesterday and I was able to put it in today. The lock nut would hand-tight about halfway and then needed a tiny amount of torque to continue. Somebody put a different type of lock-nut on it -- it doesn't fit in my valve adjustment wrenches and the other nuts do. There's some history here. It would be interesting to see what happened previously.

I was able to replace the nut and got that valve adjusted. The car appears to be running well now.

Youtube helped with compressing the spring in place. I thought this was pretty smart -- I did not compress fully -- just enough to get the locknut threaded.

https://www.youtube.com/watch?v=BcNLiKIA9hE

Dkr.