There is a number stamped into the firewall directly to the rear of the rocker arm cover. I can't remember if it's the model number or VIN. It's kinda hard to see you may have to wipe it off to see it.
Mostly high-line vehicles have the hiden VIN's, usually underneath in an area that involves cutting. Like I said: I sawzalled mine, and in pieces less than 2 foot square, I didn't see any numbers.
Remember, swappings VIN's is a big NO-NO, it's a federal rap on top of any state infractions. Why do you ask? I have seen cars that have THREE different VIN's on them! Talk about a pain to register!!! I almost bought a car at lein, but saw no VIN on the firewall--a little scraping removed the bondo and revealed a VIN different than the dash, door, and engine plate. Had to have the chassis verified and turned in all the other plates for destruction. HEY- it was a rust-free 71 CA car for $125. With a Japanese 5-speed in the hatch! Suffer...
by engine plate I meant the plate on the strut tower on older cars. on a 280zxt you are correct, it's behind the battery and held in place with plastic rivits--total of four. I forgot! Same sort of plate.