Have welder but no clue how to use it properly. Keep meaning to go on an evening course. :-/
I was talking to Mrs. KW's uncle last weekend. He's trying to save a disco at the moment (well, wondering whether to get it road legal or off-road it until it falls to bits.).
He's had issues with the sills both sides, and the box sections on the rear of the body that sit on the chassis.
Once you're sure you've cured all the rust give the underside a steam clean and a coat or two of a damned good underseal. There used to be a product called "carlife rubbercoat". Not seen it recently but it sticks like sh!t to a blanket and is rubbery so stones bounce off rather than chip.
I also have a pump up waxoyl spray doofer you're welcome to borrow.
Kevin