would be cheaper to replace with another rb30 head
however if you swap looms, sort out some custom piping, turbos, engine management
maybe injectors pending powah levels
all RB30s are the same with the only difference the oil feed/return tapped or not
depends if its a series 1 or 2 motor, however all 30s from r31s are series 2 so you can say Nissans are better
an RB25 head would be MUCH cheaper then a rb26 head but then you will need to spend extra to replace the FUGLY intake manifold
It does however require a little more modifications, in which the link below will explain in great detail
http://www.skylinesaustralia.com/forums/R3...ion-t15420.html
The PDF has most the basic info you'd need