You are not going to just be able to plug the 87 harness into an 84 body. The 84 is wired differently than all the other Fieros.
C500 (the plug near the battery on the 87) is near the center front firewall in the 84. The 87 uses a power distribution post under C500. The 84 uses the starter positive battery terminal as a power distribution post.
I would recommend modifying the 87 engine harness to plug into the 84 body harness with the exception of adding the power distribution post to the 84 body. GM realized that using the starter terminal as a power distribution post wasn't a good idea.
If you are ok/good with wiring you can do it yourself. If you want I would modify the 87 harness at a charge.
[This message has been edited by phonedawgz (edited 02-03-2014).]