best body for road race setup

Depends on what kind of racing you're doing too. Aero won't matter at all for autocross. In fact, for anything under 100 mph it will pretty much be a non-issue, and even then you'd need sustained speeds over 100.

Smaller cars would be better for autocross, not as big a deal for road courses. Even A bodies are "big" on a lot of autocross courses.

Barracuda's like autoxcuda's are a good choice, small wheelbase and fit bigger tires than the Darts/Valiants. Dusters would be good too, probably the widest tires you can run on a stock A-body, you can get 275's all the way around without any modifications or cutting. Might be able to pull that off on a '67-69 barracuda, but you sure won't do it on a Dart/Valiant. And the non-fold down seat dusters have all the same rear bracing as everything else, so, fast back style body with bracing.

Overall, I think that the body style will make a lot less of a difference than the driver and suspension set up.