400s were notorious for blowing head gaskets when they were worked hard, even with the extra coolant passages. A neighbor had a dually pickup with a 400 and hauled a big cattle trailer with it. We got really good at swapping head gaskets! The Chevy Class C motorhomes with the 400 also had chronic cooling problems.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!