There was a round retainer ring that SOR used to sell, back in the day, for a standard floor shift conversion. There was also a special, compact retainer ring used in 1974s that still had a 3speed instead of a 4 speed.
Both of those are hens teeth at this point. I’d rather not promise to get pics of them, as I am several weeks behind on picture taking already.
Interesting. I think I have a rare bird with the 4-speed tunnel with the 3 speed tranny. I have to double check - I didn't think the 4 speed boot fit, and the 3 speed fits much looser than the pics you guys posted.
If your tunnel has threaded inserts for the retainer, that are too close for the standard 4 speed retainer, then you need the unicorn I described. I’ve only seen two of them in person, one of which is attached to the hump it will be sold with.