You've gotta cut the single down tube out and replace it with an upside down Y down tube and also the 86's ran shorter front shocks so you've got to cut those tabs off and weld in tabs that allow you to run the longer 87+ front shocks. I believe you've also gotta add spacers in the rear for the rear linkage? I think. Something like that. JayTater can clear that up for sure.
I'm not sure of everything. Those are just the things I know off the top of my head