The frame on my 47 1.5 ton was rusted through or pitted deep all along where the deck sat but good everywhere else. I ground it out and filled it with weld and ground and filed it flat. I used a rod welder at about 90 amps. A modern mig welder would be better. This was not difficult.
I removed two cross members by drilling the rivets. This was to fix some rust. Local rules allow me to bolt them in with high tensile bolts. If any inspector asks I will act dumb, it was done years ago, lost in the mists of time.
Use heavy zinc high build primer. It hides rough finishes and protects well.
Main issue I had was if a rust hole went to the edge of the frame then the weld would cool and shrink and pull the frame out of line. In these cases cut out the bad bit with a jigsaw (hard on the saw and you) and cut a fresh patch the same shape from quarter inch stock. Bevel the edges 45 degrees and weld in. This is the easiest method for any big hole, rather than laying out multiple fillets of weld.
I found my frame wasn't hard to fix, just a lot of work.
