Slapper bars, will work fine until your torque output combined with final gear multiplication overpowers the short front MP spring segments leverage ability vs the cars transfer weight on said axel.

The result when that happens, is extreme rear body lift bouncing off its axel arc limits thereby creating a porpoising effect.

Your car with the powerglide vs it weight will be more forgiving with slapper bars vs a 727.

Your cars running great, but if you get much faster with more power/torque. You will see first hand the limits of the slapper bars on the short front spring segment Mopars. So far youve done great IMO With some good adjustable rear shocks you may be able to take those slapper bars to even faster ETS. mike