For CJ5s, 1955 - 1975, frame rails are the same distance apart front to back.
1976 - 1983, frame rails widen a bit at the door opening.
1982 - 1986 CJ7s and CJ8s are wide track (56" wms - wms). 1972 - 1983 CJ5s and 1976 - 1981 CJ7, and 1981 CJ8s are narrow track (53" wms to wms)
CJ5s, although produced to 1983, did not change over to the wide track in 1982 like the 7s and 8s did... (except for one legitimate production unit that I know of)
As harleydragon said, fenders 72/up are 3" longer at the back edge, than the 1955 - 71 fenders.
1955 - 1971 CJ5s have some subtle differences that can help pinpoint the year, in addition to VIN/serial number. Details like shackles, speedometer face, front (and side) marker lights, cowl battery box, wiper placement, windshield glass height, fuel tank location... etc.
Good luck... we're expecting a ton of pics soon...