Had the door and the rest of the car looked at by a body guy that knows Triumphs. Body is amazingly free of rust, so is frame in the TShirt area and where trailing arms attach as well as elsewhere. Other than the outside surfaces, the car is original paint - door jambs, underneath, trunk, floor pans, etc.
Regarding the gap - my body guy says it was likely like this from the factory. There is no evidence of body work, accident, etc.
Compression 150 on all cylinders. Misc stuff like ignition switch is funky, Speedometer doesn't work. etc.
Headed out to buy it now