'73-4 doors are heavier and have extra bracing to protect occupants from side-impact crashes.

'70s (and early '71s) have provisions for the clip-in style catwhiskers as mentioned.

While I'm sure that there are a host of year-year changes from '70-'74 - external dimensions/appearance are all the same and they are fully interchangeable.

Hardware, latches and regulators will work in all, though as mentioned early regulators & glass are different. (11 hole v. 9 hole). This may have something to do with power windows....)