I had the same problem on my 400 with aluminum heads and angle plugs. I put the boots on all my plug wires and they actually touch the headers (some of them) I noticed if I keep the temp down and good air flow they are just fine on the headers. I noticed this by disconecting my electric fan and watching the plugs/boots and they started to smell warm. I then started the electric fan up and the airflow immed. cooled them to the point where they were just fine. Maybe you want toa good electric fan that puts out serious airflow. Just a thought.