[brownbear]

I am very hesitant to weld where you cannot get at the backside to clean and paint. At a very minimum you will want to be able to after pour/spray some corrosion proofing oil in that spot to seal it with a waxy oil

I had some rocker repair to do, I was so against welding into a closed area that I cut slots in the top to be able to clean and paint in there.

Another option to welding is using structural epxoy for attaching body panels.