I've always read that heat range is determined by the ground strap. A change in color around half strap indicates the right heat range. Your plugs make it tough to determine but I'm guessing they are to hot. Here is an article I found.
http://www.angelfire.com/fl4/pontiac...Readplugs.html
You could screw in a plug a couple heat ranges colder in a cylinder and see what happens.
I need to check mine! I did on my old engine and don't recall checking this one thoroughly.