What a bummer. So the shims were completely disintegrated? Too bad there wasn't anything left to take a measurement from.
If you saw that much deterioration of the body shims be sure to check through your windshield frame/birdcage thoroughly as it seems your car has been subjected to a fairly corrosive environment.
The link Bob posted is a good one and has as good a process as any. Unfortunately, it'll probably be a fair bit of trial and error to get it right. Be aware that the body does have some rigidity to it. Don't assume the body is sitting on a shim. Depending on how much you're trying to "flex" the body to close a door gap you may find that the body isn't in contact with one or more shims. That said, if you're car is stock from the factory you really shouldn't need to flex anything as long as you replicate the original shim packs pretty closely.
As Vito mentioned you'll want to make sure the frame is dead level (front to back and side to side) and sitting on blocks before adjusting your shim packs. You can probably accomplish what you need to by leveling off the main frame rails. However, I would recommend verifying some of the other measurements while you're at it to verify no major collision damage exists.
Here's a link to the frame dimensions Vito mentioned: http://www.corvette-restoration.com/res ... 0specs.pdf
Also, here's a link outlining how to shim a C2 body. The process for a C3 is probably pretty similar. You'll need Microsoft word, or at least word viewer, to read this though.
http://www.ncrsmac.org/garage/Shimming% ... 20Body.doc