I admit I didn't know from top of my head who designed the 400. We all know it was done by the design house Pininfarina. The most Obvious name that jumped to mind was of course Paolo Martin since he designed the well known Fiat 130 Coupe. I knew however he wasn't responsible for the 400.
It took me more effort than anticipated to find out who it was. But I finally got ahold of the responsible designer. It was Leonardo Fioravanti. The same man who was responsible for the Daytona or the 288 GTO. See alo his Wiki page:
Unfortunately I couldn't find a picture of him in combination with a 400, so you will have to do with these 2 pictures: