Valve / Piston Clearance in 383

A lot of what you're experiencing in the leave IMO is in the rear suspension. That car makes enough torque to warrant 002 and 003 SS springs in the rear. They will aid tremendously in straightening the car out off the line. Also as moper suggested, have the front end aligned. That in itself can add to the ET and MPH because if the car is scrubbing the tires down the track, it could certainly be faster.