Same width. All 60-64 Full size cars are basically identical underneath.118" and 122" is in the rockers and the floor itself, mostly. The package shelf doesn't care. Someone will correct me if I'm wrong, but the cars are also wider, too. Similar to, say, a Coronet and a Monaco in 1966.
118" Dodges and Plymouths have shorter rear footwells and rockers.
122" and 126" Dodges, Chryslers, and Desotos are identical lengths unibody, but the 126" versions have longer bolt on stub frames, fenders and hoods.
Main interior floors and trunk floors interchange between all cars 60-64.
60-61 has a larger trans tunnel, but the 62-64 floors will fit in fine as long as you aren't using the cast iron trans.