Being that that is a 54 cab it will be difficult to find a donor cab that is in much better condition. What you have there is typical and how you repair it depends somewhat on your overall restoration plan. Are you looking to make a driver out of it? Are the floor boards and inner cowl panels going to be bare or covered with carpet?
My 54 was in similar shape. I welded in sheet metal to the toe board to repair the holes but used the inner cowls, inner-to-outer cowl panel and the short outter cowl repair panels to fix my 54 cab. The prices vary from supplier to supplier but most came from Chevy Duty. Be warned that these panels are not direct refits necessarily and some take some significant modification to make fit.
Tools you will need to perform this include a MIG wire feed welder, drill, good metal drill bit set, sport weld cutter (not the saw type), angle grinder, grinding and cut-off wheels, and patience. Lots of patience.
I had never done a patch panel repair previous to my effort and beieve anyone with the tools and patience can perform this work.
If you farm this out the cost would be prohibitive.
The other option is that if everything is going to be covered up, you can remove all the current rust, treat it, and either install homemade patches over the rusted areas or fiberglass the heck out of the areas to seal them. Once it is covered up you are the only one who knows or cares.
Good luck.
