converting the drum brakes to disk.
Does the bike still have the original frame? These are known as "OIF" (Oil In Frame, engine oil carried in a large-diameter frame spine tube) to differentiate them from a "dry frame" (engine oil in a separate tank).
If the bike is OIF and assuming you want a Triumph disc-brake front end, read the recent thread Using a Triumph Front Disc brake on a OIF A65
Otoh, if the bike is dry-frame, essentially the same process just the lower fork yoke is different.Rear brake
Triumph triples and twins were fitted with rear disc from '75/'76 respectively. However, they also had left-foot-shift, right-foot-brake. Not impossible to work it on to your bike but ime the pain far outweighs the gain.