Not that OD isn't desirable on most any Triumph, but I've long questioned the "need" for it on a GT6. I put many miles on both '70 and '72 GT6s and never felt that the engine suffered in any way at highway speeds. After all, 70 mph in a stock GT6 works out to right around 3500 rpm. While maximum horsepower was developed somewhere well north of 5000 rpm, maximum torque came usually around 3000 rpm.
Sure, an OD gearbox will drop the revs and save some fuel (and maybe a bit of engine wear), but it'll take years for the OD to pay off in that sense. And yes, it's also a bit more fun to have more gears, but that six is strong and flexible enough that it doesn't really need it.
Often as not when a GT6 was factory equipped with OD, the diff. went from a 3.27:1 to 3.89:1.
I guess it's all what you really want; I never "missed" having it on my GT6s. Spitfires and Heralds certainly can make good use of it, though, as their gearing makes for much higher revs at highway speeds....