I don't have that much Pantera specific experience and my project isn't on the road yet. Most of my experience is with race cars. I have read that P-cars do suffer from some chassis/body flex. Does a rust free car in good condition flex that much? Evaluating the chassis, I can see why there is chassis flex, especially with wide tires and lots of hp.
We found chassis flex caused unpredictable setups and compromised transitional behavior during cornering. I agree with David 100% about how diabolical unwanted rear steer is. We went to great lengths to eliminate rear wheel steer. Make sure your rears never go to toe out through their travel. If chassis flex causes that then that's a problem.
Good shocks are huge in chassis dynamics. If you are building a high speed car, don't overlook the spring/shock system and aerodynamics. I'm sure a flexible chassis doesn't help.