I have only flown AA, so I can't comment on specifics for Delta (tourist card purchase). I haven't flown through Atlanta, so can't comment on time between Delta flights there, but have had about the same time between AA flights at widely distant gates in Miami and it was OK. Even had time to grab a sandwich to take on board. The airline usually either sells passengers the tourist card at the gate or checks the one the passenger bought online and stamps the boarding pass as ready to fly. (The boarding pass also serves as your proof of health insurance in Cuba, so hang on to it.) I'd check with Delta though.