My feeling about head milling on GMC heads is, only cut enough to ensure a flat surface. My reason is that these things are getting a little hard to find these days, and the more you cut off the more you increase the chances for warpage and cracks in the exhaust seat area (Achilles heel of GMC & 235 heads). Yes it used to be common to take .090 or .100 off these heads but they were cheap and easy to find. Pro head work on a GMC can easily run $1000 or more, why take a chance on trashing it? Plus you introduce problems with the pushrod cover holes not lining up. Pop-up pistons are expensive but a much safer way to go.