The pad at the front of the right head should have two stampings. One will be the engine code and should contain a T for Tonawanda. It will tell you the date of assembly (month and day, not year) and the original application for the engine (manual trans, auto trans, horsepower rating, etc.). The second number will be comprised of the assembly plant code (S for St. Louis) and five or six digits of the original vehicle's VIN. With plant code and engine code you should be able to get a pretty good handle on the identity of your big block.