best body for road race setup

Mattax said

That's funny Steve, I was going to post the opposite. The reason is that the fastbacks have better weight distrubution and are more rigid to begin with. But you're point is very valid. So there's the pros and the cons. I think the final decision depends a lot on the class and type of racing.
The earlier As are about as light of a starting point as you can get. On the other hand, some of the E and Bs will get track width (which as fundemental of physics) and potential for better aero.

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I think the fastbacks are less rigid. There is a open cavity in the middle of the crar. No diagonal braces, no crossbar/package tray, large non structural glass. The back area trim panels and fold down seats also add weight on top of the rear glass. Yes, that all helps the weight balance. But it's just adding rear weight up high. You could just as well add lead bars under the rear seat and get the same net effect.

66-67 Chargers had lousy aero in nascar. That straight fastback back just lifts the back. The coupe 66-67 satellites dominated them. For autocross aero is not really a factor. But on a road course at 120+ mph is.