Not sure if this info helps, but I thought I'd just throw a number out there for you. When I purchased my custom wheels from Budnik, I paid to have the rear wheels narrowed and they charged me $200 per wheel which was actually a pretty good price compared with other quotes i got. I'm pretty sure they do it to the hoop before the center section of the wheel is welded in, so I'm not sure if the machinery that is used to do the narrowing can handle a wheel that already has the center section installed like yours. I would think that it could, but you never know. Anyway, I know that info isn't of much help, but maybe the price might be something you can compare with. Good luck!
-Chris
www.69CamaroProject.com