I wonder what Pontiac did? I saved a 1969 shaft from a 400/400 Firebird and I don’t believe it is offset. I always thought it had something to do with the pinion angle under load,but not certain it would have ever “fixed” the issue and that is probably why we saw the factory traction bar. It seems any F-body with a big block or a four speed always suffered the can opener effect under the rear seat after the leafs got a little weak. The slapper bars were almost a necessity if you put on drag tires.