The 2wd and 4wd regs are totally different, they will not interchange as the 4wd bolts on and the 2wd is secured by bracket and pipe.
The whole rail goes on no problem with correct fuel reg though.
The spacer is and issue depending on which cams you use, as it will restrict the power if you don't have enough volume and wild cams, why swedish plenums do so well, not the shape but the volume.
tabetha