I know this topic has been brought up several times, but when it comes to factory ride height, use the original 1968 Pontiac Service Manual dimensions. They list the height from rocker panel to ground for front and rear.

Nearly every 67-69 Firebird I've seen (and 2nd Gens too) ride WAY too high. Most all of these cars have had their springs replaced over the years, and Eaton never made the correct springs, just generic ones. 9 out of 10 times when someone asks the spring manufacturer why their car sits so high, the response is always, "Oh, they'll settle down, all new springs do that."

But they didn't do that from GM, and they didn't sit on the showroom floor that way. Spring manufacturers use this excuse because they don't want to do the research, and add the extra cost, to produce the right springs.

Also, many people grew up in the 1990's and 2000's, and SUV's dominate the roads. The raised look of an AWD vehicle appears normal, so they believe their old 1960's cars look "correct" if they sit that way too.

Another great resource for 'stance' reference are old magazine road tests, magazine ads, and even some old TV shows or movies that used these cars.

Here's an old road test pic, showing the correct GM stance.