Recommend me a converter for this 273 - 727 combo

68 273's have the large torque converter register so if it's a 68 engine (providing it's the stock crank) your in luck that it'll work with a 360. Also keep this in mind. You need to know what year your transmission is because 66 on back has a 19 spline input shaft and a 67 up has a 24 spline input shaft. Since you don't really know what you have you really need to take the trans. out and check the register size and input spline count before you order something, IMO.

BTW: the small register is 1.55" and the large register is 1.81"