I think you are kind of missing the boat on the thinking of jetting down a 750 carb for use on a 273. Its not the jet size thats the issue, its the capability of the carb to flow fuel AND air. A smaller carb with smaller throttle blades, venturies, etc... will make the car have crisp thottle response and run better than a carb that is too big and does not proudce enough vac. signal on your engine. Unless you are planning on turning your 273 some ungodly RPMs, I think a 750 is too big. Id say common consensus would suggest a 500 to 600 CFM carb.