Backspacing on the rim makes a big diff. Now I can possibly understand hitting the wheel wells but the A-arms??? Well, if they're hitting just ever so slightly, then I guess you can't avaoid it.
I don't know what your specs are on the rims, but I think they should fit if they were right-- I would think.
I have a friend who has a '86 Mustang and has 16x9s all the way around and no major problems like you're having. I'll try to find out tonight what type of spacing is on the rims he has (if he knows) if you don't find out b4 I do.... or if no one else here knows. In any case you may need to find out that info b4 you get the new rims....