Yes, wrong size as in the replacement parts do not fit correctly. I've heard that both from other people, and had it happen to me. The bushes are supposed to move easily on the shaft, and the gear move on the bush. Then the bush is supposed to be slightly longer than the gear. I went through 3 or 4 of each, then finally wound up reusing the old one in one case, and lapping the new one out larger for the other one.
Sorry, no idea on who has the right ones. I got the impression that this is one of those situations where all the parts originally came from the same source, and it's simply luck of the draw whether you get a good one or not.
The top hat breaking was a common enough problem that the factory started using steel instead of bronze in the late TR6 boxes. ISTR they also used the 2-piece steel bush for awhile, perhaps in early Stags.
Excess float on the mainshaft can cause problems, which in turn could be caused by the big circlip breaking or popping out of it's groove. I would also double-check that the alignment dowels between gearbox and engine are in place, although that's not a common problem with TR3s.
Check the TRF TR6 parts book, it has a page on mismatched gearbox components. Friend of mine had a very persistent problem with popping out of 2nd, that was eventually traced to mis-matched components being used by a rebuilder.
I don't know of anything else, but I'm still a neophyte when it comes to transmissions. You could try giving Herman van den Akker a call; although I don't know if he is still willing to talk about TR boxes or not.
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