One of those things you hear a lot from people who haven't tried it themselves... I asked my machinist (experienced race engine builder) if I needed to deck my 5.9 Magnum block to work with Cometics. He said not to worry about it, factory finish is fine. Engine has been together with ARP studs for 7 years and at least 10k miles, zero leaks. I don't think Cometics are really necessary though considering OP has iron heads and boost level is pretty moderate.