I have John's overdrive conversion in my TR6 and am very satisfied with the results (J-Type). I, however, went about it the hard way, by necessity. Living in Louisiana, delivering the car to Connecticut was not an option. So, I pulled the tranny myself (by myself - not easy for an arthritic 57-year-old-at-the-time) and built a wooden coffin for the UPS guy to pick up. Waited anxiously for him to return to my office with my prize, reinstalled it (this time with a neighbor, who vowed to never help me again) and it worked - once. John was at wit's end as to why it refused to work and very apologetic. Did every thing in the world to make it work and it simply refused. I even offered to fly John down to help me. That offer was politely refused. Went to our annual British car show (won first!) with a dead overdrive. You guessed it, I pulled it again (alone, again, naturally), rebuilt my coffin (UPS destroyed the other one) and shipped it back. (Hey, we are talking complete tranny shipped here) John could not find anything wrong with the unit! I insisted on another one and had him reinstall a different J-type on my tranny, which was a recent TRF rebuild. UPS showed up again, cursing me for such a heavy box, trying everything in the world to destroy this one so maybe I wouldn't ship again with them. Re-installed, drives flawlessly. The whole process was a royal PITA but I now enjoy cruising 70mph at 2500 rpm. Lovely. Oh, and by the way, the column shift that everyone sells, including Quantum, is made in Taiwan and works backwards, on is down, off is up. Read carefully in any original owner's manual and the opposite is true. Your choice is reverse the wiring (I suppose) or file the little housing to flip, which I did. The first one fell apart after one week. If you can find an original from a used TR parts dealer, you would be better off.
For the life of me, I still cannot figure out why the overdrive works in 3rd. I know, I know, it's been explained but it still makes no sense. Even less sense, the A-Type also works in 2nd. Whatever.
Bill