You can find the VIN stampings on the block & trans housing. Engine #'s should be on the front of the block just below the head behind the PS pump... Somtimes on the pass side in a similar location.
Trans#'s (if I rember correctly) were stamped on the driver side just above the pan somewhere.
As for accuracy.... I can't help ya, obviously if the #'s match your vin then they are correct. But if you're unsure of your car being an actual GS 455, instead of a clone or transplant.... You can tell what engine the car came with in 72 by the VIN....
Practically everything you want to know about a GS.....
http://www.v8buick.com
I have an old GS club newsletter that details VIN decoding, but can't look it up till Tues.
If you want, Post your VIN and I'll see what I find. Otherwise, I'll see if I can find the website I came across a few years back that had that info.
EDIT: found a site with buick production info & vin decoding:
http://www.atlantabuick.com/GranSport.htm