Sounds like Brent has you on the right track for some seasoned Twin X experience contacts.... Probably like the Columbia, they may have some weaknesses. They are a rare piece. As I understand they may freewheel... which is one feature I wouldn't like. If I'm incorrect here please know I don't have any personal experience on them.
I just know they are enormously rare.. and would be cool to own one if you have the chance.
Having rebuilt a number of Columbia's and running one in my own '40, they are a beautiful overdrive. The planetary set acts like a huge flywheel and just effortlessly pushes the car along.... I'm also running a Ryan in my '30 Tudor with a 3:54 gear set back from the '80's . When I installed it I had the 3:54 in the car from when I restored it early 80's, and thought I'd try it and if it didn't work out go back to the 3:78. Sure you have to shift early once in a while but I have a strong engine and Snyder HC head and it does just fine. Have logged many miles including several trips to Colorado, Pikes Peak, other mountain roads and passes and the 6 speeds has been wonderful to have. I was of course on the BR Tour this past spring.
After having installed and ran the Ryan, a number of Mitchell overdrives, and the Mitchell trans... in other folks cars, My best opinion for the Model A is the Mitchell overdrive...as I find it just a little more natural shifting a stick shift than a cable for the Columbia in the "A". It's part of the aura of the car for me. An option would be to use the electric shift valve (B3X) from John Connelly and have a switch on the dash or gear shift. They also need a clutch lockout for the shift valves. I have dealt with a number of the Mitchell Trans and they are good as well.. but honestly I personally like the feel of the stock Model A trans. In my opinion with the overdrive, it allows the car and you to cruise much more comfortably with less engine RPM's , water pump/fan, etc and everything else so much in addition to gaining a few miles per hour on the freeway. That said... anyone who installs these should make sure the rest of your mechanicals is top notch, steering, brakes, front end, wheels,tires.. etc
Just my 2 cents worth!
Larry Shepard