I think you should be fine with your dash assembly you would just need the dash harness and gauge cluster/panel support* from a 86-87 gn. Also would need the light/dimmer defrost switches as the harness has them integrated up by the I/P.
*not sure if the panel support assm is the same for the 81 as they were for the later years; this is what the harness was attached to from the back, and the cluster was screwed to from the front, and this was fastened to the dash. I have one of these if you need a pic.....