[chopper]

One of my Cobra books has a little blurb on the introduction of the 427 prototype at the Nassau race in (I believe) January 1965. The writeup states that the engine was a " high performance aluminum block 390". Perhaps that's the rig to which you refer?

The article contains one of my favorite photos. It's looking down the frontstretch from a slightly elevated position, and shows the cars coming at the camera with the Cobra about to enter Turn 1 and the two Grand Sport Corvettes just emerging from the turn at the far end of the straight. There's about a quarter mile of distance between the Cobra and the two Grand Sports. The caption reads "End of Lap 1." Unfortunately, the car DNF but sufficiently impressed those present with its potential.