87 Wiring Harness vs. 86 Harness???

[tex_waco_usmc]

Ok, please forgive my ignorance. I feel like a newbie, but only because the trucks Im normally working on are the generation before this one....

The truck I am building is an 86 GMC, but I pulled the complete harness and drivetrain from an 87 GMC with TBI to install in the 86. How much of the harness from the 86 doesnt need to be swapped out with the 87?

What I am getting at is: am I going to have to gut the cab and completely rerun the entire harness or are the two harnesses mostly the same minus a few changes here and there?

Someone please save me some work! LOL