How much off the block?

With J heads you are not dealing with any worries about quench and as you say there's some logistical reasons to not worry so much. Have him cut .020 off it and call it good.

As far as the value of square decking... Yes, it's much better in most cases, a little better in some, but always better overall. It's much more than a few thousanths... It's more like .002" on one end, and .017" on the opposite end because of errors in the factory and subsequent milling attempts. Square decking brings the head surfaces into exactly parallel witht he crank centerline. That's critical when you set up a quench distance, or are looking to balance power on all cylinders because they can be tens of thousanths off square for a variety of reasons. Factory machining of all parts was really horrid.