The stockton wheel web site was pretty good, here is their description of widening wheels:

Stockton Wheel cuts the center out of the steel wheels you provide, or the company will use its cores, when available. Often a center from a 14-inch rim can be used to build a 15-incher using Stockton's new outer wheel piece.
The center is tapped into place to provide the backspacing specified by the customer. Because of outer wheel piece design, there is a range of backspacing each size rim can have (see backspacing chart). Then the wheel is checked with a dial indicator to be sure the center is installed "true" to the outer piece.
Once the center is positioned to Stockton Wheel's strict quality standards, it can be welded in place. Harley welds a small area at a time and checks the wheel with a dial indicator when finished to be sure the heat didn't distort his precision work.
Now the wheel can be painted or powder coated and is ready to roll down the road. Besides sneaking a slightly larger tire under the rear of late-night race cars, Stockton also fits plenty of Pro Streeters and street rods with "Big 'N' Little" steel rims for a stylish look.