Frame
The most crucial difference between the 850 and the X is the frame. The X was designed with the up-and-coming US Federal crash standards clearly in mind. Only the X and a Volvo passed, out of all European and US cars. This is not a myth or an urban legend, it's true.
I personally find this to be comforting. The only down side is the added weight. It would have been underpowered even without the extra frame weight. If only they had dropped in the marvelous V6 Dino that Fiat actually made for Ferrari to their specs for homologation, it would have been a giant killer.
The main problem with this notion is that this was a gas-crisis car. Gandini was chided by the Fiat management of making a "poor man's Miura," but Agnelli himself was behind the project and it made it, after a somewhat extended development cycle.
The 850 is a nightmare in the frame department, with no shortage of horror stories; cars breaking in half while in motion, arriving on the US shores with rust already developed, just to list two. I would never own an 850 for this reason, the frame is a crap-shoot, no matter how nice the car.
The 850 was a beautiful car, very well styled and executed. It gets included in that era when carmakers actually cared, and it showed. They were passionate about making great, interesting, and highly functional cars as opposed to merely cost being the driving factor. After all the criticism and jokes and "Fixed it again...." junk is said and done, Fiat cared, and that's exactly why we're here as a group of enthusiasts pondering these cars, enjoying them, modifying them, and being part of a community of like-minded individuals.
I'm excited to show pictures of the 79 in a few months with her makeover. Just my two cents.