Floor Pan Replacement - where to cut?

Thank Hell_Fish. If I understand, I should cut the existing floor ~1/4" below the upper seam and overlap the new floor pan there and weld it along the seam. That seems stronger and easier. I might beat the existing pan in a bit so the new pan sits flush on the sides.

I don't want to overlap too much because I don't want to trap bare metal where I can't prime. I understand there is a special weld primer one can spray to help protect blind areas like that. Where can I buy? The new pan has some type of grey primer. Does anyone know if that is similar? Otherwise, I will first coat inaccessible areas with Rust Destroyer since good to 800 F.