The metals and designs of those old British trannies probably won't withstand the power needed to go 200 mph and especially the impact loads of long drive chain oscillations. They were sort of marginal when new. Really, everything downstream of the engine sprocket can be modern like harley based stuff from Baker Drivetrain, S and S, or Andrews. Harleys beat carp out of their drive trains so anything that works on them will be spiffy on a Triumph.
