Sounds like he's been beaned one too many times!!
Anyway, no, there's nothing on the Chassis Broadcast or the Body Broadcast Copies that specifically state "Z28". The engine code is listed in box 152, and there are all the other tale-tell indicators, but you have to know what you're looking for and how to decode them.
However, I doubt either one was found on the gas tank. Norwood was VERY good about cleaning up the cars and only a handful of Norwood CBC's or BBC's have been found.
By the way, BW code is a 4.56 12 bolt posi, but we would need a picture of the axle code to see if it's original to the car. If it is, then that's a good indicator that the car started out as a Z28! Also, does the car have the Muncie speedometer hole in the firewall?
Ed