The specs are in the Pontiac Service Manual. The Firebird sat much lower than most people realize, and most restored Birds today sit up WAY too high. But...it's all personal preference. I think the reason the old Musclecars have "risen" over the years is because of the use of incorrect aftermarket springs. In most contemporary books and magazines, most Birds sit way too high. I like to use original road tests and factory photos as a reference, as well as old TV commercials, TV shows, etc.

Pontiac Service Manual

Measure from the bottom of rocker panel, where the door gap is:

Front rocker panel to ground - 7.0"
Rear rocker panel to ground - 6.2"

Yes, the rear end sat lower than the front when they were new, that was the style back then, but I prefer a level stance.