ok so, thanks for all the feed back and the companies you have used. I have done allot of shopping.
I finally landed on an insurer.
interestingly it is also NRMA classic (well its actually SGIC vintage but same same)
they went through what the cars has and all of the details and this is what I came up with:
1989 gtr 120k (haltech, wheels, exhaust, stereo, coilovers)
$30,000.00 agreed value
windscreen and glass cover
roadside assistance
choice of repairer
$44 a month
$300 excess.
I can drive it 10 times a month and must be garaged safely
im bloody happy with that price and the fact that they are happy to cover me for what I think is a good amount for my car.
don't let your gtr be uninsured. I would use my car as a baseline tbh, oldest standard model possible with minimal average mods in good condition.