This might increase your 15" tire options.
Go to the Tire Rack and plug in these sizes: 225/65R15 225/70R15 235/70R15 255/60R15 255/65R15
If you can get your hands on GM FullSize 2WD Silverado/Tahoe/Suburban Wheels, you can safely go for 275/55R15 or 275/60R15 tires, as well as the ones mentioned above. Some have tried 275 and wider on 9C1 rims; I just think that the sidewall angles would get ridiculous if realworld handling is a concern.
If you opt for a Wheel Vintiques version wider than 8" and tires to match, you can wind up with a pretty [email protected]$$ set of meats while spending less than you would on bigger rims alone, even if you decide you need to speak to Bryan Herter to correct your speedometer.
I'm with Biker6 on this one - I want 17 inch "cop look" wheels with a little less sidewall on my 9C1.
If you're keeping the 15" wheels, 255/65R15 is the closest to stock circumference. That's important only if you don't want to get the PCM reprogrammed. Unfortunately, the only choices Tire Rack lists in that size are Pirelli light truck tires or non-performance Goodyear all-seasons. Not exactly a huge selection there.
Re:'light truck' tires for the B-body:
With a curb weight between 2 and 2.25 Tons, a towing capacity somewhere between 2.5 and 4 tons, a live rear axle (8.5" and 9.25" were available), a full frame with supsensions sytems for the bumpers, recirculating ball-and-links steering, and steering stabilizers, I think the B-body is one of the most aerodynamic trucks that I have ever seen. Besides, the Tire Rack hasn't received a single complaint from Pirelli Scorpion users.
As far as stock circumference is concerned, I know that AutoXers and drag racers have used wheel'n'tire combos that were shorter than the 27"-tall 255/50R17. I wonder if their ABS lights complain to them. Stock for the B-body is anywhere between 27" and 28"-235/70R15.
If I could afford it, though, I would appreciate 17" rims that look as close as possible to either the N97 "cop" rims, or their wider cousins, the 2WD FullSize PickUp/Suburban rallye rims.