Chrysler introduced the 300 in 1955 with the first Hemi engine (331 cubic inch displacement, not the 426 later version) where the 300 stood for the brake horsepower. This was a magic number that launched the concept of a factory made muscle car.
GM used this 300 horsepower as the dividing line between a base engine and a high performance engine. If your car was shipped from the factory with an engine that made 300 horsepower or more then it got a heavy duty power train to support that added power. In a full size coil spring car this equates to dual upper control arms.
Since the most powerful 283 only made 283 horsepower with a solid tappet cam and mechanical fuel injection it didn't make the cut. This limits you to the 327 (introduced in 1962) or larger engine as having dual upper control arms with the 1963 mechanical fuel injected, solid lifter, high compression engine was factory rated at 375 horsepower in the Corvette. Most likey you would never find that installed in a full size Chevy though it was on the engine option list. To hit 300 horse in a big car you put in the bigger 348 or 409 (or later BBC engines from the 325 horse 396 to the 390 horse 427).
Panhard bars in these full size cars are non-adjustable so if the car is lowered (most cars from the mid sixties to early seventies are already lowered two inches due to spring sag) the rear end is pushed closer to one side of the car than the other. This means if you use a wider than stock wheel and tire combination, you risk shaving the side wall of the tire on the fender lip.
If you buy an adjustable Panhard bar it will come with polyurethane bushings already installed.