Frame welding repairs are fairly straight forward, being thicker gauge metal, structural stuff, and mostly hidden after assembly (not a cosmetic problem except for concours show). If you do any cut and patch work on the frame side rails you may notice some stuffing material inside. This is primarily for sound deadening to suppress ringing vibration and acoustical echo inside the tubes. Leave this stuff in place as much as you can. It may be fiberglass or rock wool, definitely something heat resistant to tolerate the original welding processes, so it should not be burned out during you welding repairs.

The frame box sections are fairly well sealed up with original construction, except for a few small drain holes in the bottom of the side rails. Rust mostly attacks the outer surfaces of the frame, sometimes to the point of perforation. The hidden inner surfaces of the frame tubes are usually not much affected by rust, except where it gets through from the outside.

Once welding repairs are finished it is good to mechanically clean the frame using wire wheel, sanding, Scotch-Brite buffing or sand blasting. You can use chemical paint stripper applied to visible surfaces, and rubbing with wire wool may help to remove old paint. Keeping the frame mostly right side up during chemical stripping is a good idea to reduce intrusion of chemicals inside through the bottom side drain holes. It is a BAD IDEA to drill holes and have the frame chemically dipped for any reason. If you did that the fiber packing material inside the side rails would tend to hold the chemicals inside rather than draining out, and that could cause chemical leaching and outer surface paint problems later. It is okay if you want to drill small holes to facilitate internal application of some corrosion inhibitor like Waxoil. If you do that I would advise plugging the holes with plastic snap plugs after painting.