Regardless of engine all Z11s built at Norwood [N in the VIN] have Z11 body tags.
Three 396 engines were available: L34 350 hp, L35 325 hp, L78 375 hp. The L78 could have also been ordered with L89 aluminum heads.
If the car does not have a/c check the location of the heater core tubes. On a big-block the tubes are in the center of the heater box cover next to the engine. The factory punched two holes in the cover and placed a collar with rubber gasket over the tubes. If the car was a small-block the tubes are next to the cover, nearly behind the engine.
Another indicator is the fuel line. If there are 2, the car was built with a QJ carb and could not be an L78.
Things being how they are today this is all academic. A bb Z11 needs original paperwork to be taken seriously.