My neighbour, who owns a restores many wonderful cars, runs an original concourse Triumph Stag and never once has it overheated or boiled up. He gets really annoyed when you say "they had overheating issues, didn't they?" His view is that the owners had overheating problems as they never maintained them properly, knowing that the cooling was always a potential issue!! I like that view point. What he means is that if you keep it clean and use good fluids, there is no reason to have cooling issues - and he doesn't have issues.
I've toyed with the idea of a Stag for a while. They do have their foibles, like any car of that era, but much of their reputation for unreliability was down to poor or badly misunderstood maintenance requirements. The engine (for its day) was quite sophisticated and makes a lovely V8 noise for such a small capacity lump.
As mentioned earlier, the BMC 11 / 1300 series claim to fame was that ALL body panels were carried as replacement parts. The Stag also has its own unique attribute. It was the only car to be specifically excluded from aftermarket warranties - they really were that bad at one point!