If the bolt were the same length as the head is wide, the bolt's new angle would be as far away from original as the distance milled: The bolt head would be moved over .160", etc. How "loose" a fit the bolt is in the head now will determine whether the hole must be enlarged; if it's 9/16" now (bolt is 1/2"), that's only 1/32" radial clearance and the bolt will bend in the head.
I'm not sure .160" is safe, even if only taken from the plug side (which I assume was the idea?).
