Hawk, that's a great diagnosis page - excellent pictures.
Looking more closely, the 350/420 piston shows some chipping (to the left in the picture) whereas mine did not. The head gasket problem seems to match up pretty much spot on. Where the piston in the 420 looks burnt away rather than chipped, it may have started out chipped then burnt away. It all happens real fast once it starts! That would also match in with little or no high EGTs. It would also match in with the fact that there looks to be very little aluminum in the exhaust port. Mine was absolutely coated in aluninium but the 420 has less. It may be that a lot of the material departed in chunks (from "chipping" and only a small amount burnt away.