While you are doing this, a lot of the factory chassis measurements are shown in the factory FSM = Field Service Manual. I'm sure it would help you verify your chassis straightness prior to adding reinforcements.

I used the US Cartool chassis kit on my '68 A-Body convertible and added a home-built Monte Carlo bar, which really eliminated the little cowl shake that still remained. Initially I installed the subframe connectors and drove the car for a while. I could really feel the improvement in chassis rigidity from adding those, but it also really made the cowl shake noticeable on bad roads.

Once I added the rest of the stiffening kit, the majority of the cowl shake was eliminated, but it wasn't until I added the Monte Carlo brace above the engine that all signs of cowl shake disappeared. I never did reinforce the rear frame section, but I don't feel any movement back there either.

The biggest help on these old uni-bodies, is starting with a rust free, never-been- hit chassis. Unfortunately, most folks don't live in an area conducive to these cars remaining rust free.