The .44-40 is the most correct cartridge, I have an original 1873 musket from 1891. But .45colt is fine in a modern ‘73. But be aware that some ‘73s are very cartridge oal sensitive so being able to switch between .357 and .38 spcl length may not be possible on your rifle. I have a Stoeger ‘73 that will run .45schofield loads and 45colt but my other uberti will only run 45colt length rounds. Might want to check it with some dummys.