Later model 460 heads had a bump cast into the exhaust port to direct the air from the thermactor air pump. some trucks were smog exempt and didn't have the air pump but they still used the same castings with the restrictions,they just didn't have the holes drilled. I don't think the C8 heads have the smog bumps. if you have bumps you can grind them out for a few more horsepower. the old ford factory performance catalog showed what and how to do it I don't remember which year they changed the rocker arm design or when they went to hardened valve seats for unleaded gas.