Which body for best performance?

If we're talking 67-76 like the OP said then the only post cars are the 67-68 Darts and '67-69 Valiants. Barracuda's were notchbacks or fastbacks, there were no "post" Barracudas 67-69.

The bit about the post cars being "stronger" somehow is "old wives tale" territory for me. Lots of opinions, literally no data or evidence that I've ever seen to back that up. Unless you've seen the structural analysis between the different body styles it's just a guess. There's just too many differences between the body styles to say that just the addition of the post made a lick of difference between the 2 door sedans and say a Barracuda or Duster.

And of course, the addition of any significant frame stiffening or a cage will pretty quickly make any difference from the post body style pretty much moot.

If you like 2 door sedans, awesome, build one. If you're going to be racing at a high enough level that you might notice the torsional differences between a 2-door sedan A-body and any other type, well, you'll have a full cage and won't notice anyway.