Generally, it isn´t useful to lighten a 280 Z shell in a way, that even a scrutineer without perfect knowledges of the differences won´t recognize this. The structurell important differences are easy to discern and above of all these aereas are the important ones for creating the strengthning difference of a 240 to an 280 shell. All the not important parts, which are possible to lighten without weakening the structure are existing at the 240, too. So, there is less sense lightning a 280 shell to the weight of a 240. then better use a 240 from the beginning.
Also we have to take in account, that it is not allowed by the policy of the FIA to use the rollcage for strengthening the shell as it was usual and legal in the SCCA. By the way. Due to this the design of the front dome bars shown at the red ralley car in the thread "New" Group 4 Racer is only half the rental fee. Best would be to connect this point inside the cabine to the connection point of the rocker panel - rear wheel house.
Even yet I couldn´t understand were the mentioned 300 lbs Phantom was speaking about, are taken in. Sure the doors are conspicious heavier parts. But 300 lbs more weight for the 280 shell, when the complete 280 Z was 500 lbs heavier than a 73 240 Z at all???