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X-frame cars are desirable for their styling (thank you Harley J. Earl) but hated for their lack of a frame or a functional suspension. Bill Cosby joked about how comfortable it was to drive a GM X-frame car, spinning wildly down the road allowing you to watch the world go by sitting on a comfortable couch and viewing everything through a broad expanse of curved glass.
As this article explains there is no one person who can be put against a wall and shot for the design. But it was influenced much more by artists (body design), and marketers (ad-men from the Madison Avenue business schools) than engineering.
Automotive History: An X-Ray Look At GM?s X-Frame (1957 ? 1970)
Though the marketing people claimed the central spine as used on the old Elan from Lotus, and currently used on the C6 and C7 Corvette, is strong (it is if it is big enough to resist deflection under load) it was more important to make it smaller to lower the floor for more passenger comfort (which yields a flimsy easily bent out of shape foundation upon which the car is built on top of). It was designed to bend and roll under which is okay because when the car was designed the fastest you could drive in America was 45 mph (which is as fast as you can go with stock brakes).
A half century later, three generations after the original designers made their decisions you are comparing this rolling living room to the performance of a BMW or a Lexus. There is no comparison. So you will be disappointed.
The aftermarket that sells parts are marketers and salesmen. The parts they sell can kill you if you put the wrong part on your car (your survivors are welcome to sue the manufacture of the part in China). As a mechanical engineer I look at these parts and wonder if any one gave them any though at all in designing them. People who know nothing about Pascal's law, or Hook's Law, or front suspension design happily buy a part and put it on, and are surprised that it affects the handling designed by thousands of GM engineers who made it that way after evaluating thousands of other factors. (mostly costs as determined by the bean counters).
The 1958-'64 full size Chevy was designed to look great, but the earlier 1955 Chevy would run rings around it. The economy car (Chevy's original economy car) the 1960-'69 Corvair handled better than many English sports cars) so it isn't that Chevy didn't know how to make a car handle but the wrong people were driving the company's decision making during this period of time.