Which body for best performance?

Depends on what kind of performance you’re talking about –
If you want to go drag racing, or stoplight to stoplight, probably 67–69 Dart or Valiant post.
If you wanna go road racing or carving up the canyons, Probably a 67–69 Barracuda notch or E-body Cuda or Challenger.
If you want to do Bonneville or the Texas Mile, Probably a 67–69 Barracuda fastback.
No matter what you decide to do with it, if you want something like a modern performance car, it’s going to take some work.