Which body for best performance?

For A body cars, the post cars are stronger body wise.
The other issue to think about is wheel base. A shorter wheel base is less desirable by some. The longer wheel base cars will be better for drag racing. This is obviously seen in the extreme at national events with dragsters.

The longer wheel based MoPars like a Charger are really good at going straight and fast but with a weight penalty vs a shorter wheel base that’s lighter (less metal) but can be a hand full when it gets loose.

FWIW, I’d look for a suitable Cúda and run the roll cage ASAP along with frame connectors to start the show with. That will stiffen up the unibody right away and pretty good. You can take to the next level from there.