It is indeed Fangio taking the Corvette SS for a spin at Sebring in 1957. Though they didn't race the Corvette, Duntov was keen to validate his new baby by letting Moss and Fangio take the car out for a few laps. The reason this photo is not elsewhere on the net is I took a screen capture of a film clip.
Now for the sticky part. whcgt's first answer names two drivers - hence my question "Which driver is pictured?". Multiple guesses are typically frowned upon. I had intended to lock it for whcgt, since he clearly knew what he was talking about. Then D-type chimed in. I was going to give a point to both, so nobody would go home empty handed. Then D-Type changed his mind, saying it wasn't Fangio, which is wrong. And then whcgt zeroed in on Fangio.
Point to whcgt