Try posting this question in the Tag Team section.
You need to find the numbers/letters stamped (not cast) into the engine stamping pad. The casting numbers will only tell you part of the story. From the information you have given so far, all I can tell is that it is at least a 396.
What proof did the seller give you that it was an LS6 454?
Many long years ago before everyone got so picky about matching numbers and originality I worked the counter at a very large Chevy dealer, and I helped a lot of guys do "Badge engineering". A lot of customers were slapping 454 emblems on 396 cars, even if it still had a 396.
Best of luck, Thomas
"Bomber" '67 El Camino, Beater comes back to life.
Was 350/TH350 14.90 @ 93mph, 360,000+ miles on car
Now 406 roller, 340rwhp, more hp coming, 3.08 gears
Street radials, left in drive, 13.20 [email protected]
8/1/01 added Plum Mist '67 to collection
*New* 468 on its way - going Big Block!