What a lot of articles about "prototype" cars fail to mention are production methods. For instance the "AMP" car which is the 3rd from the top in the images Jack posted (It still exists and has been restored) used a tubing frame. It was good for judging size, layout and to a limited extent, styling. Compare it to the late prototype model which is the 4th image. There are styling cues in common, but if you compare frame and body construction methods, they are quite different. The production HJ was revised to use as much stamped steel as possible.
One of the Brooks Stevens proposals for a post-war Willys car was to use MB Jeep frames with conventional passenger type bodies. This would result in an odd looking short coupled body. But more important, the frame would be too stiff, too heavy, and much too expensive in terms of material and assembly.