Roush 427IR spark plug hole stripped

[169mph]

I pulled the spark plugs to inspect and replace last month. I had hell removing four of the plugs. All the plugs were over tightened in my opinion. I had to use a thread chaser on three of the plug holes. The first two went smoothly. I was very careful and I used a borescope to make sure no metal shavings fell into the cylinders / pistons. On third hole, which is the #1 cylinder, I cannot get the thread chaser or a plug to thread in. The threads look pretty bad. Should I pull the head or is there another option? Also, I saw carbon built up on all the pistons but Im not sure if it’s normal or not. The piston below was the worse.