I used hard block in my 408c. It helped eliminate cap walk and is supposed to enhance ring seal by keeping the cylinders from distorting. I noticed a little bit of a cooling issue, but nothing major. It just runs a like 5-10 degrees warmer than it used to. But I think the pro greatly outweigh the cons.

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1982 F100 2wd-408c CHI 3V 225cc Heads
675hp and 576 ft lbs @ the flywheel on pump gas