I think you're ahead to to modify your receiver to be honest. Finding a good gunsmith is very difficult. Many gunsmith these days are like the quick change oil place's tech folks -- can pull an oil plug, but ask them to weld or time w/ a timing light and they're SOL. Same goes for gunsmith's if it's more than just a cleaning and basic part swap...
Rather than gunsmiths, I'd look for a quality builder that's well experience in milling.
First one that comes to mind is from the galil world and that's Jeff Miller/Tennagalil/Hillbilly Firearms.
http://hillbillyfirearms.com/There are many, many pictures of his work over the Uzitalk galil forums and probably elsewhere on the interwebs. He's built a few VZ58s too.
Here's a recent one:
http://www.uzitalk.com/forums/showthread.php?75386-Latest-Aquisition-from-TennGalil
He does lightening cuts on Galil receivers for R4 builds, engraving, all sorts of various finishes. Highly detailed.
I'm sure there are others, but my advice is to look to receivers with experience building/modifying milled receivers -- FALs, possibly 80% lower AR builders (though a lot of them just freehand dremel), Galils, M14/M1 builders are the ones that come to mind offhand.
