No garage used to working on modern stuff is going to weld up something that old, as said. It's not worth the hassle to them and they'll either refuse to do it, saying 'it's too far gone m8' or quote some ridiculous figure. Why would they bother, when they can charge just as much to do timing belts on Mondeos etc.
It's restoration work.
I've had great success with mobile welders in the past. These guys know any job they go to will be more extensive than the owner realises and they will have seen it all. You can increase your odds of a decent job further, by sourcing replacement panels, cleaning the area first etc etc. It'll most likely be cheaper, too.
Really sounds like the owners of this car have had the frighteners put on them, it may be 50p sized hole... or complete sill replacement, but I reckon it's well savable in the right hands.