it is a combination of things poorer fuel , leaner mixtures, making more heat & a thin casting , it is typical even for the earlier 308/302 castings but it is easily fixed with a plug & new seat , I still prefer the Shaft rocker to the far weaker GM style Ball stud system which is why I like the 308 casting better , or you can spend more $$ & buy Alum heads but you will need to use a small chamber head or change pistons to bring up th ecomprsssion to compesate for the heat loss
