milling 360 heads

I have taken .060 off my J heads BUT you will run into mating problems with the front and rear seals and the intake. You can safely take off .030 without any real problems as long as you use good gaskets- but it won't raise the compression much -maybe .3 to .5 points. As for finding out if they have been cut before your machinist should be able to measure the head thickness and compare to stock specs.(measure from block surface to top of the valve cover gasket rail)