Dolomite 1850 with 3 rail gearbox used same clutch as 2L Vitesse. My 2L Mk1 Vitesse has a 3 rail j type overdrive gearbox, reverse up by first, with the later fine input shaft so uses the clutch from the later Dolomite 1850 single rail gearbox.
Triumph never made this type of gearbox, but some rebuilds have, so the only way to be sure is to look.
The only difference is the friction plate.
Regards
Paul