I had the exact same problem with my truck when it was brand new. One of the extremely small freeze plugs was not set correctly and it leaked antifreeze. My dealer set a new plug with out removing the head. It is possible, Snap-On makes a tool for setting the plug.
BTW, I have a 56 series block. I think this is a common problem. To see if there is a pattern, I have Engine SN# 56789879, what is yours. maybe there was a run of these with similar ESN#'s.
-Rich