512 Build

That piston is 2618 aluminum, which requires a little extra clearance for the higher expansion rate. I would err to the large side of the tolerance specs at 13:1 compression, a little piston slap/rock is a lot better than scuffing and cold seizure. If building tight, I like the looks of the Mahle pistons with the skirt coatings and metric rings, but they are pricey, P/N 120375F08 on this list. http://www.us.mahle.com/media/motorsports/bbm-web.jpg
Mahle shows piston clearance of .004-.005. Here is the list for Icon from the friendly folks at hughes, notice the drag race specs, which is what I would think a 13:1 512 would be, .006 to .008.
http://www.hughesengines.com/Upload/productInstructions/iconinstallation.pdf