Guess I will jump in here too. As with The Tool Man, I am new to the z car, so trying to learn what I can here before tearing into the work that my car needs.
I installed weld-in subframe connectors on a '69 Camaro convertible. It was by far the best improvement I made to the car. Wrestled for a long while about cutting/welding in something so permanent, but eventually drivability won out for me. Once installed it was like driving a completely different car! I did not realize just how much flex that car had until the connectors were put in. Combined with some suspension tweaks and PU bushings, cornering and handling improved a ton. That thing drove like it was on rails.
The subframe connectors I used (and I assume The Tool Man also) were tube steel (1" x 3" if memory serves) members that were welded into grooves cut into the floor pan. They ran from the rear of the front subframe to the front mounting points of the rear one, and were also welded to each of these.
If there is something like this available for the 240z, I will be all over it. Have not found anything yet.