I assume this is a track only car, so...
A 1200x1800 sheet of polycarbonate (Lexan) cost me $168. Enough there to do the rear quarters and rear window, and its fairly easy to do yourself. I saved around 10kg
For the doors, i'd be taking out the power windows and gutting them (side intrusion too if you have a cage) 30-35kg there
Get rid of the heater system and put in a heated windscreen. Another 10kg
Take out the Fuel tank and replace with a cell (less weight + less fuel weight) up to 20kg total.
Defiantly do the bonnet/ boot in fibreglass or carbon. (hmmm, isn't the GTR bonnet alloy?)
If you've got money to blow and your regs allow it, maybe single turbo setup???
Go on a diet. easy weight saving if you are a big guy.