milling 360 heads

I am sure this has been talk about a million times,but I take a lot of stock from everyones advice.My 915 340 J heades are in the hands of the machinest now.What I would like to know how much can be milled of my heades to bump up the comp.Thing is I dont want to run into any problims like milling my intake,or haveing to get new push rods.I would like to mill them and use my stock push rods and rockers and intake.My power plant is a 360.Thankssss.