New to me 89 420. Replaced the Bosch WR8DP spark plugs that were in the car with Autolite 64's which came up as a direct replacement spark plug. The car started and ran fine for about 45 minutes and then a complete loss of power. Pulled the plugs and plugs 4 & 8 had the electrode squished to a gap of.10. The timing chain is stretched to the point that the left side mark on the cam pulley seems to be out by 1 tooth. Replacing the timing chain and valve stem seals is planned. What caused the spark plug gap to be squished and should I be looking at something else before spending the bucks on replacing the timing chain? Compression is good at 140-150 psi on all 8 cylinders.

Thanks!