The modern rim isn't sectioned in the narrowing process... unless it is a mag. If we are talking about a pressed steel rim, the outer hoop is removed from the center and replaced with a new hoop of the correct size. The center of the rim can be welded back in at almost any offset you wish.
Anyway narrowing or widening wheels is very expensive. If you can find a set that will work without the customizing, you will likely be ahead. I have used Stockton Wheel, since they are in the backyard of the San Francisco Bay Area. They weren't cheap, they definetely weren't fast, but they did an outstanding job on the rims I brought them. These guys have been widening wheels since the days of horse drawn carriages. They did right by me and I would use em again without hesitation.
John
