gear venders

Playing with Triumphs over the years, I've gained some experience with the Laycock overdrive. Good unit, works well. (MG, Jaguar, Volvo and others used these units extensively as well.)

Gear vendors has adapters for a lot of common transmissions, and they've been selling kits for years.

If your budget allows, it's probably worth considering. Just make double darn sure the unit locked out in reverse.