Top of frame just in front of A arm. Wire brush and wet it.
1965 ID plate Rivets were supposedly "Rosette" style. Yours are not. If you still care, describe or post pics of rear end suspension components. That could determine Chevy VS GMC. It's not a huge deal. A collector will not want it but it's been thru DMV, I assume, and no hicups. Lots of trucks have been re-bodied with Chevy parts and a few have been re tagged with a title and tag from something else(illegal unless thru DMV). Something is way off. Maybe nothing bad though. Reason for frame pics: GMC may be different than Chevy a little bit. I'm in touch with a person who can identify with pics. Not me.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn