I might make an exception for the Boss but I'm not sure. Chronologically it's barely a '70s car.
Ex-wife had a Mustang II of similar (within 3 years) vintage and it was semi-rolling train wreck. Something about a "known issue" with the ignition wiring that they didn't share with the masses.
Every time it rained, which in Rochester NY is always, it refused to start. After about the third visit from AAA I noticed the problem could be solved with nothing more complicated than a shot of WD-40. It's at this time that I learned that "WD" was short for "water displacing".
I wound up buying a case of WD-40 and dealing with it. Never did get around to replacing the hydrophobic wiring as it wasn't covered by Ford's shitty warranty. I should have bathed the entire car in the stuff as it attracted rust like Mongo repelled women. And I'm pretty sure Ford sourced the powerplant from Maytag. And not a very good Maytag at that once the smog gadgets were added.
Any exception made for the '70 Boss would be, perhaps inaccurately, tainted by the molestations they inflicted a mere three years after. As such I consider the '70 Boss to actually be a far less disagreeable 1960s vehicle. And it was sold in 1969.
When it comes to shitty American 1970s vehicles the Mustang Eye Eye was more representative of the malaise inflicting that woebegone decade (sold 73 through 78). The Boss was well and truly dead through much of the '70s.
Technically, the '70 Boss would be an exception but not in the true spirit of the benighted piece of shit they provided through most of the decade. The '73 / '74 example was sufficiently plague-born that I considered the 1976 TR-7 we replaced it with to be an upgrade in every sense of the word - and if you're familiar with the TR-7 that says a lot.
The guy that came up with the Mustang II should be drawn, quartered, stabbed, and then hanged. Not even having a Charlie's Angel driving one on tv (Kate Jackson had the Cobra II model) made that piece of recycled Maverick leftovers worth driving. [well, she made it into a chick car at that point, at least in my mind. ]
The EPA and safety regulations that bombarded the car makers starting in 1970 caught them flatfooted and slower than a deer in the headlights. Their response was to simply lower the compression on their behemoth V8's until they had the same horsepower as today's riding lawnmower.
But you'd never notice the loss of power, seeing as how just about everything on the car would break 5 seconds after driving off the dealer lot. By 1974, the damage was done and Japan was beginning to take over the US auto market in terms of reliability and certainly gas mileage.
Indeed the '70s were a dark time for the big three. The American ingenuity I had come to rely on was largely buried under a tsunami of stodgy thinking and unimaginative engineering all while lobbying was considered a more viable route than engineering one's way out of a problem.
This was demonstrated by almost everyone else on the planet managing to meet our requirements while still providing an automobile worthy of the name. Until we wake up in the 21st century to learn that cars.com listing of "most American content" includes two Japanese names.
Perhaps the best thing about the '70s is that they ended and should remain in the dustbin of domestic automotive nostalgia. On a brighter side they eventually recovered. First in fits and starts until in '08 an automotive reviewer noted that "it was the first time in living memory that we've used the words "Cadillac" and "World Class" in the same sentence.
In fact that CTS was introduced in a manner that nobody familiar with Cadillac could have guessed at a couple decades earlier with the unveiling to the press taking place at the Nürburgring. A 1970s Caddy would have spontaneously combusted if brought within ten miles of Nürburgring even if covered with a tarp.