There is a 6 speed gearset for other Triumphs, but it's anyone's guess as to whether there is enough metal in the twin case to machine it out to fit. There is also the problem of adding another position and cam slot to shift the extra gear.
I have a feeling that the way to go would be to fabricate a steel trapdoor with a 6 speed gearset attached, similar to the way Sportsters have been since day one. In this case, a second trapdoor may be needed on the right side. This way, you would machine out the entire side of the gearcase, flange, drill and tap to bolt in the trapdoor(s). Much easier than flycutting the case to fit around the new gears and stronger too.
All you guys making parts for these bikes, if you have a milling machine, this might be a worthwhile project. Get an couple of engines from wrecks and swap the 6 speed into the twin.
