We used to grind all the crud away till we found solid metal to work with. Used a plazma cutter to cut the bad frame out leaving solid frame to weld to. Wire brushed clean and I forget the steel gage thickness now, but it was compareable to the thickness of the frame we were working on. Had a metal brake to bend any thing we needed to bend to fit the cut out areas. Used weld through primer on anything being welded in. Used our stick welder back then, long before mig was ever around. Ground down all the welded area and back then sprayed Dupont Imron two part epoxy paint on all the areas we worked on. Then sprayed a rust proofing like Ziebart through a long hose into the boxed frame to stop or at least give some life to the repaired frame. I did a lot of foreign cars back then and the Datsun and Fiat were know rust buckets and the frames rotted before the bodys did. We kept many of them running for years by patching the rot. I thinking now that my biggest customers were Mercedes and and Jags too.