I thought you had a gauge for the 3.0 L and needed an adaptor for the 3.5 because of a different sized injector?? In this case, I was thinking that the glow plug openings would be the same. In any case, for sure, you would need to remove the manifold to get at the glow plugs for performing the test on the 603. Clearly not as easy as the injectors. I would think that you need to contact the manufacturer of the guage for the proper adaptor.