I installed a set on my K5 back in the early 90's. I worked for a police department and every year we were replacing cars, so I had an unlimited supply of 6" lights. Install is pretty easy. The main thing you need is the install kit with the correct post bracket. It comes with a metal sleeve the the drill rides in while you're drilling out the post.
Remember there are different lengths of shafts for the lights. The Unity catalog says your truck should be 14.5" center to center and 9" exposed shaft. I know on my K5 I used a set out of a 1990 Crown Vic. I liked those better because the center to center is only 13" with a 7.5" exposed shaft. That way I wasn't banging my hand on the handles. I don't know/think the CV lights would work on an 81-87 dash as those stick out a little further on the driver's side..
https://www.unityusa.com/Application-Search_ep_77.html
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