Get one for the head, it has the most sensors and will probably be the one most needed for work. And I would suggest getting in for the car, since it is connected via the harness to the head and there for you will need to be able to trace wires and since you have a GSR the suspension, axles and what not should be the same, so the only thing you'd be short on is the block (which is fairly similar to the B18C anyway).