I've got corrosion and a patch needs welding on both sides near the rear seat belt mounts. I plan on removing the plates and welding a patch in there the re attaching the original plate to the new metal. Just wondering whether the weld needs to be seemless or I can spot weld in various places? Will determine the size of the patch I put on.
Many thanks.

Fully seam weld everything.

You can buy a replacement section for a few quid, well worth it.
Flush fit the part and seam weld