Why dont you remove the fuel tank, make a custom tank which mounts in the spare tyre well in the boot. And replace the tank under the car with the gas tank.
Then you've got everything you want. Plenty of fuel to start the gtr should hold about 45 litres (I made a sub enclosure and measured it with Litres, its 48 from memory, you may have less because it would have to be less for fabrication)
Hope it helps.
Removing the spare tyre hole would make it illegal, surely it is a important part if you get rear ended, how often do you see them bent? Id say only when they are wrapped around a pole... If you had no tyre well the pole would be where the gear stick is and glass would replace your neck hair.