If I was to shoot from the hip I'd say 1-5/8" would be best, maybe even 1-1/2" if you can get them. It depends on what kind of tracks you'll be racing at. Stickier tracks, like Beaumont, require torque to spin the tires enough to get the car to turn, where slick tracks like Willis will need a smoother torque curve to keep from blowing the tires loose (I'm assuming you'll be racing without a wing since you're on this site asking for advice). Another consideration is whether you'll have to run mufflers or not. You might give Smiley Sitton a call and see what they're using.

Tim Simmons