Absolute first order of business? Reinforced frame rails from BadDog.
While this Z won't be seeing a lot of track action, the torsional rigidity (or lack thereof) was what made the previous build so unpleasant to drive. These rails fit perfectly. There were two small areas where the original rails had been dinged and deformed, but heating the affected areas with a torch and returning them to their original shape allowed these rails to slide cleanly in place.
I decided to bridge the connection between the rails and the rear subframe framework. Keeping in mind that none of this is empirically-proven or even supported by anecdotal evidence, it IS an area that would seem to benefit from being tied together. Some 1/8" heavy steel strap was formed to evenly match the profile of the area to be bridged, tacked into place, and then welded in fully. It's not pretty, but pretty is for the top side.
I also added some triangular gussets to the corners of the rear suspension box. Again, if it helps, neat. If not, we'll never know.
The design of the S30 suspension and unibody make sit a little less-likely candidate for the "strut tower brace" that you're used to seeing on, say, a modern strut-suspension car. It simply doesn't benefit as much from tying the tops of the towers together. However, the upper frame horns relative to the lower rails leaves a little to be desired. I recalled from my MazdaSpeed Miata build that "frog arms" were a hugely-effective modification, and I suspected they would help here as well.
The suspension on the S30, much like the MIata, is cantilevered out from the front bulkhead (which, on an S30, is terribly weak). If we can support the upper frame horns, my theory was, I can transfer some of the cornering load into the bulkhead, the A-pillars, and the lower frame rails. If not, nothing of value has been lost, and I got some much-needed practice with my welder.
1x3 rectangular steel tubing with gussets at each end (to match the profile of their tie-in placements) were fabbed up and tacked in place, then welded properly. A small L-bracket joins the brace to the inner fenderwell about halfway up.