All you need to know about the 1973 Pontiac Firebird is that Clint owned one.  What better stamp of coolness do you need?
The 1973 Firebird lived to see another year, and two significant additions played a big part in its longevity and journey to iconic stature.  The first was a new SD-455 (Super Duty) engine that somehow passed muster with the EPA regulations.  In the face of the looming energy crisis and dwindling pony car sales, the Firebird had lost some engine power over the years.  The 350 V8 was downgraded further to a why-bother 150-hp.  The Formula 400 V8 slid to 250-hp.  The 455, available in the Formula and Trans Am, was introduced last year and hit 300-hp.  But the new SD-455 was based on a racing engine and featured aluminum pistons, oversized valves, and a specially designed camshaft.  Though listed at 310-hp, in reality it got closer to 370-hp and could be easily modified to reach even higher numbers.  Hand-assembled, only 252 Trans Ams and 43 Formulas received it in 1973.  As muscle cars continued to shed power in the new energy-conscious era, the unique SD-455 made the 1973 Firebird one of the few truly racing performance cars left on the road.
The other legendary addition was a new bird decal on the hood of the Trans Am.  Previously a small image on the front bumper, the fondly-dubbed "Screaming Chicken" splayed its fiery wings over the entire hood, dripping with gaudy testosterone and overblown kitsch.  But it worked, and that chicken has become the Trans Am's identifying feature.  This combo of race-car engine and mythic fowl helped significantly boost sales of the 1973 Firebird.