I have the same problem with #7 plug being too close to the header. My problem is that it's arcing through the boot.
I am running these Accel shorty plugs
http://www.summitracing.com/search/department/ignitions-electrical/part-type/spark-plugs/shorty-spark-plug/yes/make/chevrolet/engine-type/v8
I think I'll try the insulators that Rick suggested. I did try one fiberglass mesh sleeve from Summit and the boot still melted though it.
Any ideas on how to stop the arcing throught he boot? Have been using MSD boots as well as some hi-temp ones as well.