Looking at the floor of our 70 challenger. appears to have a pink paint under the plum crazy. Was that the color of the primer they used or was it a rustproofing. I only see it inside under the paint on the floor. I also notice it on the side of the spare tire hold down in the trunk?

IIRC a Plum Crazy factory paint job is applied over a gray primer base, and I believe the gray primer was shot over a red oxide finish, but that would be on the main outer body, the interior/floor areas would be in the dip finish primer (gray) prior to the vehicle in final paint...

so it's possible you might be seeing pink/FM3, assembly line workers would sometimes clean out their guns of the previous paint run by blowing out the color in what would become hidden areas later when the vehicle was in final assembly