85/86 interior into an 87

Lots of guys have gone the other way...87-93 in the 79-86 cars. Most of the holes are there, you may have to drill a few new ones. The only real problems are with the quarter panels, as like you mentioned, the 87-93 are deeper at the windows than the earlier ones. You would have 1-1/2" gaps from the edge of the plastic to the window. But you "might" possibly cut the parts from a 87-93 panel and epoxy them onto the 86 to make it fit better. But the actual glass that you can see through is the same size on both cars. Maybe something to look at.

The door panels might be a problem if you have power. Most 79-86 did not have power, so if you have it, then you would have to rig something for the switches or somehow make your armrest work.

Hopefully you are not thinking about the dash...it could be done, but not without quite a bit of work.

Good luck!

you might be the only person in the world who will have swapped an older interior into an 87-93

well if it works the other way around then sure u can do it.
ummm 79-86 gt's have their power windows buttons not the door not the center console like the 87-93s. also the newer console seems like it has a space off to the right side for the ebrade handle as the older console make it look like its centered. i dont know. IMO the dash is UGLY from the older ones. you will need to fab some clips for the dash. well i guess you have to cuz u need to fabricate clips if ur doing ur swap in reverse.