My 1980 3 on the tree collar was broken when I bought the truck six years ago. JB Weld and hope we’re the only things holding it together for the 40 mile ride home. I could not find a 1980 shift collar to save myself and I didn’t want to do the floor shift conversion.
So… I ordered the 1979 version and compared it to what I had. I carefully marked where I needed to trim with a scribe and promptly launched it across the room when I got greedy with the lathe cutoff tool.
So… again I ordered a second 1979 collar, marked it for cutting on blue painters tape and used a fine tooth blade hacksaw to carefully work my way around the piece. I think I used a piece of 4x4 wood in a vice but may have slipped a pipe through the center. For God’s sake, take your time. It is only a one cup of coffee job.
I dressed up the cut with light file and sanding. Quick coat of satin black paint and I’ve been happy with it for six years.
I recall inserting the pivot pin for the shift lever gave me the most stress, because it was a tight fit and I did not want to crack the eggshell thin collar after modifying and installing it. If you have a small ream, that could be the way to go. I was committed by that point so just went for it. Sorry, no photos.
Good luck.