I think those HC heads are prone to strip inside the sparkplug outlet, so be gentle putting and pulling sparkplugs in and out of them. They can create unforeseen airleaks, which would cause a mystery seize. I've used a tiny drop of ghetto gasket to correct slight leaks there.
I've come across two different sized HC heads before...one had a deeper, slightly wider lip, and the other was more shallow. They looked the same otherwise, but once I put the shallow one on, it completely re-arranged my previous tuning.