Ok, I did this conversion on my old skyline
It is quite simple, all you need is ether a s2 support (hard to find), or you need to cut a bit of your s1 support away. The bar will then fit fine.
I did it myself with a angle grinder and about an hour of time
Headlights are out by about 5mm, noone even noticed on mine, but for a full s2 conversion new headlights grill and bonnet will be required to complete the car.