Why is decking the block, not a option? Strip the block, power wash it down to remove the greasy stuff and get it decked. Cost 50-100 dollars and some of your labor. Some shops will complain that it is not magged or hot tanked. Just get it good and clean for them.
After decking, repeat the power washing and blowing out of water and spray down with WD40 quickly. It will be fine.
I routinely deck from head .030 without blinking a eye. .050 is also ok by me under the premise of not using it for heavy duty continuous loads (like a large truck /trailer). 060 would also fit into that category and not leave room for more normal decking. Racing heads can go more.
It will be hard to "measure" the thickness of the head deck. I use the factory casting marks a my tell tale. There is a thumb sized area that is unmachined on the head surface. .060 will practically remove this area.
