I run a F-Body Road Runner at similar MPH. If you find no defects in the rear mountings, I would check the front end. The front suspension is quite different on an F-Body. Is the K-Frame still mounted with rubber insulators, if so they need to be swapped to metal blocks. You can fab them, used to be able to buy them as well. Second is the center link level to the ground? Many had the problem of the idler arm bracket being located incorrectly. The bracket on the K-Frame can be heated and bent into the correct location very easily. Third the torsion bars also act as the strut rod stabilizing the lower control arm. If either end of the torsion bar bushings are loose you can induce caster and toe change. Forth make sure the steering box mount isn't cracked. The rest is pretty straight forward as far as worn parts and proper alignment. Good Luck.
Doug