How does the former owner know they were milled too much? Do the intake bolts holes no longer line up? Do the heads knock and ping? The heads could be cc'd by a good machine shop to determine the chamber volumes and estimate the amount they've been milled. There are various thicknesses of head gaskets.Stock steel shim head gaskets are around .020"; blue fel pro perma torques around .039" compressed. Sounds like you have a 2nd set of heads - the 'original' 906 heads - get them cc'd along with the others and compare them to the 2nd set. You might also mike the surface of the heads between the deck and the bottom row of bolt holes between the two sets to get an idea on the about they have been milled.