Actually there was talk of a MkIII with a 4.0 4WD Jeep drivetrain a while back, and some blurry pictures. Interesting stuff, as the R154 is supposedly fairly similar to the Aisin transmission some of them have.
Anyways, it would likely take much more work and money than you expect. With a 'normal' build, you need to add at least 10% to your budget as cushion, since you will end up having to replace some other parts like it or not. With what you're looking for, wait for R32 GTRs to hit 25 years old and just import one legally. I know all sorts of people with an R33 GTRs, R32 GTRs, and one that had an R35. As much as I respect and quite like Skylines, I have always preferred the Supra myself, despite how awesome ATTESSA is