Looks great and I love 'rescue' builds. This car could well have just died somewhere and left to rot, while the old owner may have a little to explain, you still saved this GTR and it's still on the road because of you.
For rust, on my old R33's it was always in the wheel wells and the corner of the guards (the flat angled bits near the tyres) - in Japan, they lay salt on snow to melt it. The salt then gets picked up by tyres and splashed around inside the guards. I'd assume chassis around the wheels also.
Looking forward to the updates!