Look at the 3F within cowl tag, look at cowl plate, plate rivets, originality of the VIN plate and correct roseate rivets.

PHS will tell you VIN and all the options and for 67 only, the Engine Unit Number (EUN) Compare the EUN on the front of the block.

Also..Your cowl plate will have a date code. Your engine, heads, intake, rear will have date codes and casting numbers.

You can compare the date of the body to the dates on all of the above. It is real difficult to find correct and correct dated parts to do the 326 to 400 swap unless there was a huge financial gain to be had. For instance..guy has a rusted out 400 HO or RamAir...buys a solid 250 cammer or 326 body and switches plates and VIN's. Is possible.

Essentially, if the heads, block, intake are dated 1-8 weeks before the chassis build AND the engine components all date within 1-2 weeks of each other, you typically will have a correct car. When production first ramped up early in 67, it was common to have chassis dates 8-10 weeks different than drive train. As an example, my 3rd week of April chassis 67. My engine, rear and trans are all dated mid February of 67.

My 68 400 HO 4 speed chassis built 3rd week of December 67. Rear date 12/15, heads both 12/17 block 12/17 intake 12/12, trans 12/18 distributor 12/24, alternator 12/24 starter 12/10. The exhaust manifolds are dated September and November 67. Why the difference and spread? Not too many HO's and Ram Air cars were built versus cars with log type manifolds..thus, the production dates were limited to say a 20 to 1 ratio.

My 68 400 4 speed chassis built 4th week December engine date 12/6, heads 12/09 and 12/11 intake 12/07 rear 12/03 exhaust manifolds 12/07 and 12/08.

The EARLIER the build within a model year (July-October) the greater the spread of dates between chassis and engine components.

With my factual examples, one can look at the chassis build, all of the drivetrain casting numbers and dates and determine with a high degree of certainty if a car is as it sits without PHS. Looking at a Firebird and telltale signs, aside from the obvious badging and 400 specific stuff..a person can generally eye whether it is a 400 or not.

Off my rant and soapbox, gave my .02 worth.