IIRC, the headlight openings in the 78-81 are different then the ones in the 82-87. I do know that some people have swapped the early aluminum core supports (somewhat rare) into later model g-bodies. I see no reason the 80 support wouldn,t bolt into the 86 trucklet.
You will not have to unbolt the fenders back by the doors or the inner fenders. Everything in front will have to come loose. It is not that bad of a job. some of the bolts coming through the fender into the header panel can be a bit of a pain.
there are 2 on top and several around the battery tray area where the inner fender bolts to it and a couplewhere the sides of the core support meet the fenders vertical edge.you need to remove the battery tray. there is one on each side from the bottom up
sratch the last one .i need to put my eye crutches back on sorry . there are several screws along the top and a couple on the edges where the header meets the fender edge .you reach them from inside the wheel opening with the inner fender out or you can reach through the opening where the horns are but you will loose skin that way