my head is only 137.5mm and runs greys low comp t34 and mapped at msd.

afaik ford have min thickness to avoid valve/ piston contact but this would not apply if valves pocketed. some tuners say that the seat inserts come too close to head face and others say this is not important. some say that the face of the head is hardened and that you lose the hardness by excess skimming (this is the case with k series heads) but this can only be true if the material is exotic stuff like LM25 which is capable of hardenning but i dont know if YB heads are made of this. spoke to karl about this and he says skimming the head has no effect at all.