1979 C10 Spark Problem

I have a 1979 C-10 with a 350, I have recently noticed that I had two spark plug wires that I thought were bad. I had on occasion from cylinders 4 & 6 the spark arcing to the block. Like I said I thought that the wires were bad, and replaced them, when I started the truck yesterday I noticed that the arcing was still there. I pulled the plugs out and looked at them and they both seem to be fine, except that the top of the plug where the wire sits seems to be worn a little funny. I am going to replace both of those plugs today to see if it helps. Any other suggestions would be helpful.