He bought the car with the motor in it, but the guy he bought it from specifically said it was a NAPA motor. As for if it was ATK that built it, I am sorry, but I can't be sure. I will ask him if there was any paper work with it that would say so.
If I was you, i would talk to the guys at the NAPA and have them show you all the fine print on the warranty. I am sure there is a catch in there, and am almost certain that if you have ANY mod on the car, it will void the warranty. Also, the warrenties 99% of the time are only valid if installed by a certified shop/mechanic.
Check it out.