You guys can believe what you want...and I can believe what I want. Those pictures look like a lot of overspray...not intentional painting to me. WHY would the factory waist paint and time? They were about making the most money they could per car. The entire bottom half of the car was already dipped in primer. If they thought that was good enough for the exposed underside, why would the interior need more protection?
You're absolutely right, you can believe anything you want. But as a guy that used to do paint and bodywork, the interior of these cars was not oversprayed, it was intentionally painted. You can't get that kind of coverage with just overspray, not that consistently. I can't tell you
why they painted the inside floors, but they did. I can't even tell you why they painted the floors, but didn't paint the bottom of the roof skin. Maybe Ma Mopar had an aversion to painting underneath things. Why did they paint the inside of the trunk and not the bottom of the car? Wasn't for added rust protection.
All I can say is that all 5 of my mopars, which cover 3 body styles, 4 factories, and three different model years had their floors painted on the inside. And all the ones I've seen pictures of were the same way. Can't say that makes it absolute, but I'm pretty darn sure.