I wasn't confident in any of the kits I found online and there are so many contradictory websites/listings regarding these trucks so I'm posting my results to help anyone else needing to do theirs. - my truck is a 65, don't know what years these will fit, I assume 60-66'.
What I used - 1958-1982 Corvette Headlight/Headlamp Adjuster
Kit 244005
These were described as fitting older vettes but said they did NOT fit the gmc. I have also seen these kits with nuts/bolts for installation but the easiest and probably best way would be to use rivets like the originals, also I don't think you can get to the back of the bracket to use nuts/bolts without removing the whole thing, another +1 for rivets.
I carefully used a grinder rather than drilling the old rivets out. I bought 1/8" & 3/16" x1/2" rivets because I could not find 5/32" which would have been perfect, 3/16dia. is too big without drilling. I used the 1/8" X 1/2" long rivets which were too long for the horizontal (left/right) adjuster locations without a little drilling, just put a 1/8" hole in whatever is keeping the rivets from going in any farther and they worked great. All in all with minimal work these things are nice and solid and my rust bucket can now be driven at night.
I had seen other posts where people used coupling nuts or thread extenders to replace these things, I have the coupling nuts but the new bolts that came with this kit anyways.