Thanks for the pat on the back John but I've only ever come across 2 possibilities on those heads, the long reach plugs like the R44XLS, and the shorter ones like the 44, R44, or R44S. They are both the 14MM thread. The 18MM is the size that was used in Ford FE V8 engines as one example and they were a taper seat plug to boot. If his head has been reworked to take 18MM plugs it was probably to repair some badly damaged 14MM holes as Chopped 49 pointed out?
Cross ref'ing an 18MM Ford engine plug to a 14MM GM engine plug heat range sounds like it will involve some time poring over catalogs and decifering heat ranges. Standard application listings won't be any good if that is the case.
