GM had commited itself to purchasing so much aluminum from Reynolds (or was it Alcoa?) in exchange for Reynolds to build a plant in Masena NY. GM started with aluminum Corvair engines, then aluminum Buick 215s, then aluminum Powerglides, then aluminum intakes & cylinder heads. They also need to evaluate concepts. F*** produced a half-dozen stainless steel '36 models and later some Thunderbirds. Perhaps there were a number of different imperatives that lead to the production of that aluminum 300 2 door; namely feasability study, racing (who us?), paint adhesion in the real world, etc. A quick way to get feedback on something is to release them on a limited basis & get the feedback. The 2-door sedan body was the strongest body style and that alone may be why it was selected, although it is hard to ignore what F*** had in their Thunderbolt.
Given our litigous society it is surprising that GM would release the car. My wife's cousin from Warren, Mich. has an '86 IROC-Z that the GM engineers used to evaluate the Corvette 350 in. In order for that car to be sold they had to put the original 305 back in (he put a hot 350 in it soon after). As for that aluminum 300 2-door I'd sure like to find out where that car is, too!