Max. removeal would be .060" and anything over about .030" I would start to worry about intake valve to piston clearance, especially with a high lift camshaft. recessing the intake valves is the usual solution, but because of valve geometry and valve spring height, .060" is the limit. Installing shims under the rocker shaft stands when taking off a significant amount is a good idea to help restore rocker arm to valve geometry.
As for a "stock" head thickness, I couldn't say, but I have two heads in the basement that I believe haven't been surfaced, and the hole depth of the front right head bolt is about 3.778" on both, if that helps any.
