Honestly, if you "read" the rules, it really isn't in there. I wrote the first draft of the current bodyshell rules as I was trying to make it easier for everyone to be able to mount a shell and be pretty confident that it would be legal to use.
As Dale mentioned the GBS "drawing" might be in there, I can't remember - but no where does it say in the rules text that a racers shell will be checked against those specs - they are just there for manufactures.