Rust repair

[slotard]

My '73 C20 has plenty of surface rust, including lots of missing paint, but no visible holes. Well, I was swapping my seat yesterday, pulled up on the carpet and it turns out I have some holes. I hadn't even thought to look before, it's always been a California truck as far as I can tell and with no holes on the outside I didn't think to look that close.



Looking from below I only see holes along the line the ones there are on, and a few on the other side.

Any suggestions on where to buy patches? I'm an ok welder but haven't done any bodywork. Where is a good bodywork guide? I certainly don't need perfection but holes are a different story.