Mr.Mc Farlane, cant be heads, rule out the possibility of cracks. See you should have rechecked/retightened the fasteners of cylinder heads at the proper tightening torque once it got at operating temperature - for the perfect fit, i presume you have fitted new gaskets on both the banks. Also care should be taking while fitting gaskets to replace new rubber seals joints for oil circulation to the cams and rockers, these seals fit between the cylinder and the head and it should be replaced every time heads are taken out.