I have 81-up inner fenders mounted on 74 front clip. They fit, kind of. Not all of the bolt holes line up and like said the core support is where the real difference is. Mine took a few smacks with a hammer to get them lined up so I could get at least one bolt in each side. The battery tray doesnt fit quite right either. It will either line up with the fender or the inner but I ended up have to drill a new hole in the inner fender to get it mounted.
Though you are going the other way around, Ill bet you have the same issues. Once I find some better fenders Im going to pick up the correct inner fenders.