Cleaning up the battery/core support of 70 Skylark, and the pocket for the frame mount was packed with clay or some kind of putty. I don't see it referenced in Assembly manual. In the pics I have already dug it out so I could clean and treat the rust. Should I replace it and with what?
No, do not replace it. Someone was trying to cover up the rot. Someday you are looking at patch panels for the inner fender and core support.