Not just anything that fits the ITR will fit the GSR and LS. Obviously, intake manifolds aren't swappable between the motors, as the bolt patterns are different. The GSR header won't fit an ITR or LS, because the header/cat/bpipe are different lengths. And obviously, VTEC valvetrain components aren't swappable into the LS. The reason the Type R uses the PR3 casting head is because Honda had so many of the heads left over...they threw a mild P&P on it, and some upgraded valvetrain.