I moved from the Black Country to Cornwall 30yrs ago and rarely go back so I've lost touch with what used to be local businesses. Look for a car restoration company rather than a car repair company.
It's common for restoration companies to have to add new metal to replace long extinct body parts so will have the skills and knowledge to do what you are asking, whereas a modern repair shop will usually just want to fit a new part rather than try to repair an old or damaged part. Also, they may not put the required time and effort into repairing the underside of the car when you're never going to see the repair.
If your wheel arches need welding it's going to be more cost effective to fit new panels. They'll have to be painted after welding anyway so it's something you'll need to consider.
If you find a car restoration company, go and speak to them and tell them what you want and I'm sure they'll be able to accommodate you.
Have you considered a full body respray? A good auto body shop would sort the welding during their preparation work