Yes, you could still get the 429 Super Cobra Jet in the Ford Mustang if you worked over the options sheet just fine. The biggest Mustang with the largest amount of cubes available does seem like a no-brainer, but even though the Mustang had become the largest it would ever be, cubic inches didn’t mean everything. Models like the Shelby G.T. 350 and the Boss 302 proved that a smaller, lighter, and rev-happy V8 could still trump cubic inches, plus add on a benefit of better handling due to less weight in the nose. But there was a problem: as the new car was introduced for 1971, Ford was backing out of racing. The Boss 302 had been a homologation special, as had been the Boss 351. But if there was no place to play…then what?
The answer was the street. With 330 horsepower and 370 ft/lbs of torque, the Boss 351 was a giant-killer. Got a big-block and think you’re bad? If you found yourself lined up next to one of the 1,806 examples that were built, you were about to find yourself having a very bad day, indeed. You got a four-speed manual and you liked it, you got a 9-inch rear axle with 3.91 gears and you liked it, and you got the ability to scare Z/28 and Trans-Am owners. And guess what…you liked that too. A lot. 
You could go as mild or as wild with the looks as you wanted with the Boss 351, but we really dig the simple blue/argent exterior and we really dig the dog-dish caps with trim rings…the only other option was Magnum 500s. We also like the fold-down Sport Deck rear seat…there isn’t a Mustang made whose rear seat isn’t a punchline, so at least you can turn the expansive SportsRoof ‘Stang into a funky, almost shooting brake kind of car. In fact, if it wasn’t for the fact that you’d be turning over 3,000 RPM to run 55 MPH, this would make a solid daily. But the Boss 351 was the last of the faithful, a small-block screamer that could bully quite a few cars around when pushed hard.
Mecum Auctions’ Denver 2018: Lot S159 – 1971 Ford Mustang Boss 351
When I worked in a gas station, a kid down the street from it had a dark green one of these.
I think I remember an article from the time comparing 1/4-mile times of the Bosses 351 and 429.
The 351 was quicker.
And again, if I remember correctly, it was stock-to-stock, the 429 enjoying quite a fair amount of detuning.
Regardless, canted valves are just cool anytime, on any motor.
Those are pretty awesome!