I can tell you that nice original 400 Birds are fairly hard to find. Start with the PHS as said before. If the owner has the original Protect-O-Plate, I believe the motor unit number on the block is on the card. If the 68 400 does not have the VIN or if you suspect a fake, you can start by sanding the paint off the motor unit number, two letter code on the front of the block and the VIN to see if it looks like it may have been restamped. Blocks also have their cubic inches cast into them--such as 400 or 455.

A cast date a few weeks to over three months before the build week of the car is reasonable. My Judge's original block is about 3 months before the car build week.

Of course an engine's heads, intake, carb and exhause manifolds can be checked for the original part numbers and reasonable cast dates. If some are not original, you can probably find them, but an original block will usually have its original heads. Intakes, carbs and exhaust manifolds are often changed.

I believe that most 68's had the VIN in the block--especially the later build cars. It is hard to see. If unsure, take your time and take someone who has done this before. A mirror held down from the top can do the trick.

Also, most of the charts do not show the later production 68 Firebird 400 intake part number, as it changed late in production. My fourth week of May 68 Bird has it. Let me know if you want it--a pm will ensure that I see the message.

Good luck