it's a matter of changing the shape of the inset so it is more square as you can see in the pictures. the core support doesn't need any major modification other than re-drilling some holes to mount the light, as the jdm ITR headlight mounts nearly the same way as the jdm DA headlight, IIRC only two holes per side are off by a bit. finally the hood does need some modification, the center lip where the emblem sits on a g2 must be widened, as the jdm ITR headlights are shorter than the jdm DA headlights.
