I just went out into the shed to compare doors & I agree that the sedan door will probably fit the hole in a coupe, although i doubt if it will work vice versa. The ends of the door along the top edge are diferent, & while this may not make any diference in a racer, it will look odd in a streeter as there will not be a match up in panel fit around the door pillar or windscreen pillar area. It may even be necessary to trim some metal on the coupe door to allow it to fit a sedan as the coupe door is not cut away at the ends like the sedan door is.

The door skin for the two door Sedan & the Van is not listed as a replacement for the coupe for this reason, but i'm sure a skilled panel man could modify one door to fit properly in the other bodyshell.