Uh Oh - Here we go again!
Well you will need to start by switching out the entire interior -- noting is the same. Many have had more than a few problems getting the 87-93 interior parts to line/bolt up do to the differences inmounting locations.
You may need new doors on account of the different style mirrors.
All the molding on the car will need to be changed.
You will need an 86 engine wiring harness in order to plug n play into the existing 79-85 body harness. Modify the 86 harness to accept mass air .
The hood, bumper brace/brackets, bumper covers, lights (front - stripe your current 85 tails)
Figure out how in the hell to install air bags to use a stock looking 93 Cobra steering wheel.
INstall a EFI pump and wire it up to work with your ignition.
Get an 8.8 rearend. True dual exhaust and tranny crossmember.
Swap out the front spindals and struts to use the 87 up brakes.
Sure seems like a lot more work than putting a for sale sign in your 85 and buying a 91-93 GT where you only need a handful of stuff to make it work.
Hope I helped