I'd be inclined to comment that a car with enough power and torque to appreciably cause structural problems with its unibody is already enough of a "race car" that we can no longer apply pure street-only concepts to it - regardless of whether you actually race it. Conversely, a car that is only driven in "street-type" situations probably would not benefit from a huge amount of torque, with the possible exception of rare and brief smile-inducing bursts that in themselves do not justify going to the effort of installing extensive structural mods.
Probably your best bet would be simple strut tower braces. These can be home made with a drill, hammer, vice, and saw. Beyond that, consider subframe connectors, especially if you have a 240Z.
I decided from the very start that I wanted a maximum effort drag car, and that drove my decisions on structural reinforcements as well. But perhaps people are too concerned with the example of those individuals who really do have full-effort race cars, and who really do need roll cages etc. Most V8 Z conversions would probably be fine with no cage and no braces of any kind. If I were doing the conversion alone and on a tight budget, I would not worry about structural mods, because the car that I could realisticly build would not need them.