For the same reasons I put Buick Rallye wheels on my '65 Chevelle, Cragars on a Cougar, Baby Moons on a '49 Merc and black lug nuts and center caps on my STS - it's his car and he wants it to be a tad "different".
BUT, there are a lot better GM wheels, not requiring adapters, that will bolt right on. Some of the later Deville and "DTS" 18s, for example.
- click the small pictures for a zoom. His wheel is a "high positive offset" type, about 45 - 47 mm.
The reason to keep the same (within 0.25") original outside diameter of the tires is to prevent speedo and engine management errors. Original size on the driver's door sticker, wheel specs in www.tirerack.com