I'm in a similar boat, not quite as extensive as yours. however, its isn't easy. although the metal is thick enough as i have learned that you can pound on it hard and not go too far. welding isn't as bad as the metal is thick enough to not blow through. cut a bad piece off your parts cab and take a jigsaw or sawzall and cut a slit in it. sand it to bare metal with a flappy wheel on a grinder and use that to set your welder as well as practice. get it fit with as little gap as you can, spot and dot skipping around to keep the heat down. Worst case scenario, you have some experience and need to buy a different cab. I still think you are better off buying a parts cab versus a repop cab. The repop panels i have bought aren't as right as i think they should be. I wouldn't imagine that an entire cab would be right either.
as an example, i bought new kick panel replacement patches, the originals had grooves that were angled but flat at the bottom "U" shaped... the patch panels were "V" grooves. a little thing, but still not right.
