I've seen a pic at one point in time with the SN dash and the fox door panels so I do know that you will need custom door panels, or the side of the dash will cover the existing handle and you will not be able to open it easily from the inside. Here is a link of a car that had it done, but I dont think it will be much help. http://www.musclemustangfastfords.com/features/mmfp_0610_1980_mustang_cobra/index.html
Do it man! Fox bodies arnt my favorite but I have always said if I was gonna buy one it would have to have an SN95 interior. I just hate the fox body interiors. Not many people have it so you got my vote.
its been done, thing is you need to trim the dash shorter, a fox's dash I would say (from memory) sticks out like 1/2 a foot, our SN95s stick out over a foot.