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I sent a picture of the GTX to my friend Earl….here’s what he said about it… “I think it is a Challenger with the 440 cubic inch engine and a ‘six-pack’, i.e., three two-barrel carburetors, hopefully installed with progressive linkage.   That means that around town in ‘normal’ driving conditions only the middle two-barrel carburetor is providing the air-gas mixture to the intake manifold.   Then, when the driver pushes his foot down on the accelerator, when the middle carburetor is three-quarters of the way toward wide open, the two end two-barrel carburetors start to open.   As the middle carburetor goes from three-quarters open to fully open, the two end two-barrel carburetors go from fully closed to wide open.    As you may imagine, the owner of such a car is not interested in the miles-per-gallon of fuel consumption.” I know that’s more than you wanted to know…but that’s what the 440–6 stands for. A true gas guzzling beast.

BTW, he's writing about the "sister car" to the Plymouth GTX...the Dodge Challenger...they were virtually twins except for the brand.