To answer your direct question, Hemmings always has people advertising date coded castings. Be prepared to dig deep in the wallet. You will be looking for CRV or CRR (CRS is not likely) codes on the stamping pad. The casting number is 3963512 (which should be the same as your LS5).
I am curious as to reason you would want an original LS6 (engine casting). My opinion only: unless the car is an LS6 car, you will be paying a premium for performance that could be duplicated less expensively. Good luck.
[This message has been edited by Alan F (edited 04-22-2000).]