First check the play in S/S blades and shims.
Assuming that's OK, have a good look around the chassis where the triangular gusset plates are weled in place, just ahead of the bulkhead.
My '76 suffered as yours - eventually traced to rust building between gusset plates and chassis pushing them apart, plus a crack in chassis (which may have stemmed from the St. Morgans Dance). Car now has the later, longer plates welded in.
I also have brake reaction bars, don't know that they make a great deal of difference off the racetrack.
Good luck!
