BB shells
The BB30 only really relates to the ID or diameter of the shell. The 68/73mm size relates to the width of the bb shell and really has nothing to do with the diameter of cranks that either shell will accept. There are adapters available that can be installed in a BB30 to accept either a standard 68 or 73mm BB which is 24mm. Standard crank bearings are apprx 24mm ID where the BB30 crank bearings measure 30mm.
I did read a thread somewhere on here where a guy had a machinist modify a 68mm standard shell to accept a BB30 and crank. I can't even remember if it was a Cannondale bike. I think it was a brand x bike.