Whats happening in #6 Cylinder

My engine only has 300 miles on it from the rebuild. I pulled the plugs to swap them to something different. This was #6 Cylinder on my 360.

View attachment #6 Plug (1024x576).jpg

What is causing this? I shined a light into the plug hole and the entire piston is carboned. I know I have a questionable set of plug wires on the car and I just ordered a good set up. It almost looks like the valve stem seals?? Whats your opinions?