Re: GMC 4000 pictures
 

Look good. You really don't need brackets for those. On both of my trucks they were used for Directionals and on the '62 6500, they were bolted through at the bottom of a C-channel plate welded in-between the frame rails. On my '64 C-30, I bolted them through convenient holes that were already drilled at the ends of the frame rails. The other set you have with Amber on the front and Red on the backs are typically used on the front of the truck.

You still will need some Stop and Tails as if you notice, what you have there are only 1-Wire lights. Stop and Tails will have 2 Wires.

Re: GMC 4000 pictures
 

Yes, these are the lights for the frame. The lights I bought were sold as a set. I'm still looking for replacement tail lights. You can see in the picture the frame lights. The tail lights are on the mounts. You can see the mount on the right is a piece of steel.