Best way to go about this?

I bought just the actual cab from a 1988 extended cab “rust free” from Texas with a full interior and lower miles (50k). The carpet was damp/wet and this is what I found under it, the underside of the cab is truly rust free which is why I bought it. Idk how to treat this right with the side and rear glass still in. The windshield has been pulled and I walked all around it inside and it’s solid but I don’t want to keep spreading. Some idiot put oversized speakers in and cut the metal as you can see. I’m a bit mad because look how much less rust for the most part the old one (red) had besides actual holes

Do I grind and bedline before carpet? Just brush POR on it? I bought new carpet for it already

It might not look too bad in pictures but it’s flaky