Well, the sexy approach would be to find and fit an original TR6 (optional) AC unit. Not sure how many are still around, but there have to be at least a few that someone has taken off. Main thing would be the evaporator and controls (stuff that goes inside the cabin), as you'd want to replace all the hoses and receiver/dryer anyway; while condensors (the part in front of the radiator) are pretty generic.
Helped a friend of mine start down this route some years ago, but haven't seen him for a long time so I don't know how well it worked out. But he needed a bigger alternator even before adding the AC, so I would plan on that as well. (He used the Bosch unit from a 78 Ford Fiesta with AC, which is a bolt-on replacement for the old Lucas, and plug-compatible with later TR6.)
And I still don't have the AC going in either Stag (need to get them running first).