I saw in California Bill's book that he recommends torquing the head bolts to 50 ft lbs initially, and then increasing by 10 ft lbs until reaching 90 ft lbs on the 15 bolt engines. I know the 18 bolt engines use 1/2" head bolts, and this is consistent with using 90 ft lbs of torque, if the 15 bolt engine also use 1/2" head bolts, I think you'll be safe, but double check the thread size before going the additional amount.
