Welcome to the World of Corvette, and to DC!
The Z51 was limited to coupes with manual transmissions in 1988. If you look at the center console door, you should find a "Service Parts Identification" sticker, and that lists all the RPO codes associated with your car. If the console door has been replaced, you can order the duplicate window sticker for your car from the National Corvette Museum (all they need is your VIN).
I just replaced my stock Bilstein shocks with a set of Bilstein Sport shocks. The Sport shocks are the ones that came with the Z51 package, and they made a big difference in how the car feels. The standard Bilstein shock for the Vette is called Heavy Duty. I'm not sure if Sports are available for '88 verts - you'll have to look at the Bilstein application guide to find out.
Where did the leaf spring crack? Any other damage that might have contributed to the failure? If you get caustic cleaning agents on the spring, it will deteriorate (but I'm not sure that caustic by itself will cause it to crack).