You can try Tom Smalley, he may be able to help you out with your gearbox and o/d (especially if they are already out of the car), but I can tell you that a few years ago he had no interest in working on my TR6 differential. At first he said he could do the work, but after having my car in his shop for 1.5 years he finally confessed that he wasn't interested in doing the work. I asked who he would recommend -- he named two other shops that would do the work, but he said he wouldn't recommend that I use either one. No hard feelings though -- this was at a time that I had no garage and he stored my car inside for 18 months, tuned the carbs, fixed a turn signal ground and gave me a state inspection, all for $10.
Another guy who may do the work or at least make some good recommendations is Art Lipp in Orchard Park. He races a TR4A. Although he seems to be focusing on his go-kart business, I would be shocked if he wasn't still involved Triumphs. I've got an email address for him if you are interested in contacting him.
Scott