First, I would be cautious if the seller says that the first eight VIN digits tell you everything. That said, the sure way to tell what it was is to get the get the Pontiac Historical Services (PHS) build record but that is $75 and you will need the full VIN. http://www.phs-online.com/order.htm

Note that there are body differences on 400's. However, over 49 years some of those could be changed or missing.
'68 400's got a different hood with hood scoops and "400" emblems. They also had "400" emblems on the trunk lid and a Pontiac arrow on the middle of the front bumper. 400s and AC equipped '68s got upper and lower steel baffles and rubber fillers between the grill and radiator to direct in more cooling air. 400s also received decorative chrome "hockey sticks" on the plastic grills.