Most '68s, as all later years, have a partial VIN stamped on the block down along the timing cover on the passenger side on the flat pad on the block face. This will not be a number-for-number match of the VIN, but has the division code, plant, year, then last six digits of VIN. See this link for more info: http://www.firstgenfirebird.org/firebird/FAQ/codes/enginecomp.html

There is also an engine production code, shown here, which should match the PHS engine unit number http://www.abilitypc.com/photos/firebird/uploads/engineid.jpg

In addition, 400 cars had the Pontiac emblem on the front bumper and the 400 emblem on the deck lid.