You can go here:

http://www.phs-online.com/

and for $35 they will provide the original invoice for the car which will tell you all the original equipment that was ordered. There is also an "engine unit" number listed on the invoice that you can match to the number to the left of the timing cover which will tell you if it's the original engine.

Also, go here for info on identifying the engine:

http://www.teufert.net/pontiac/engcodes.htm

http://www.teufert.net/pontiac/bloccast.htm

Good luck!