leaky heads gaskets, I'm losing it!

New info everyone. Not sure if this is my problem or not. I picked up a good used 360 magnum motor this week. I measured the thickness of the heads at the corners that are machined with the mounting surfaces. The set of heads I am trying to use is .025 thinner than stock heads. Looks like they have been resurfaced before I had them done again. How much can you take off of these things before you start having problems?