LOL I hear you on the migrating junk. Dang stuff seems to have legs.
The idea is to only cut back till you get to solid metal. There's no need to use the whole patch panel. If the battery box is rusted away at the front, you could still leave the back section in and make a joint or joints somewhere forward of that. I opted to leave the whole battery box intact, even though it meant welding a butt joint all the way around the perimeter on two sides.
My seat riser flange is rusted away on the driver's side nearly to the bead on the riser, so I'll make the joints under the seat, adding metal to the floor patch panel as necessary. I already have the new flange for the seat riser fabbed up, and will weld that in place after the floor is patched.
