I have a 1974 TITAN motorhome in my salvage yard that is now a storage unit that had the 440 engine with the extra cooling passage. They were 902 heads and did have the 5/8 hex head plugs. I believe all the 5/8 plug heads were the extra coolant passage heads. The head gaskets are available. I replaced mine as precaution even though my engine was a 49000 original mile engine. It had sat for 10 years after owner passed away and the family had me tow it. I put the engine and trans in an 86 pickup and it runs great. If you have the engine out on a stand now is the time to do it.