When you pull the head, Look closely for cracks. Use a magnifing glass if you have one. between valve seats and sparkplug holes is a common trouble spot. You need a good straight edge and feeler gauge to check for warpage. if it's the gasket leaking that's most likely the cause. There are specs and procedures in the overhaul section of most manuals. The only way I know to check for cracks not visible is magnafluxing. I really don't know much about it except any good automotive machine shop can do it. They can also check for warpage and shave it if necessary. Another common cause of leaking head gaskets is faulty installation. Before you pull the head, put a torque wrench on a few head bolts and check for tightness. If they weren't tightened properly, that could be the cause of your problems. Sounds like the 4.6 is basicly the same as the 258. On the 258 and 232 The first head bolt from the front, on the drivers side goes into the water jacket and is supposed to have sealer applied to the threads on assembly. Probably has nothing to do with it but any one putting on a head with out a book most likely wouldn't realize the hole even goes into the jacket so wouldn't add the sealer. whup, gotta go, good luck.