Hi Dick,
It sounds as if you changed all the major componets. Did you have a front end or 4 wheel alignment done? I don’t know if that will help your problem, but it will not hurt. Thats what I did afer the assembly to be sure the car tracked correctly. Also be sure the rear axel is stable. My car (chevette TD kit) handles well at highway speeds so there must be something left. Good luck.
Bill Hursh