Did you install the head bolts dry, or with some sort of lubricant? If the bolts aren't lubed, some of the torque is wasted overcoming thread friction. Drain enough coolant to keep from spilling it all over the place, then remove the head bolts one at a time , use a lube like 30 weight motor oil on the threads and under the bolt head, and retorque them. If you use a high film strength lube such as never-seez, reduce the torque specification about 5% or so.
Jerry
