Sounds like an interesting project. But an important thing to remember is that the GpA cars were compromised by rules and regulations. If you are going to build up your GTR then i think you are best to just use the modern parts and build a neat car like many of the guys on SAU already own.
Short of going crazy in the stripping and caging of the car...then there isnt much else GpA you would be doing to your car. Especially if you are only looking at using Whiteline based suspension.
Go a basic turbo upgrade. Get a good gearbox, beef up the brakes and suspension and spend some time on cooling and you have your all round performer...after all that is the strength of the GTR
On a side note there are street GTRs in Vic that are quicker then the Gibson cars, and they run RE55s, not slicks and have air con and full interiors