ORIGINAL: Super08
Some of the plugs you listed are not the same. They have the same thread but are two to three times the size of a CM-6. The The Denso U20M is the exact same fit as the CM-6. For example below is a picture of a Delco M8, it is about three times the size of a CM-6.
Post #5 has been corrected.
Thanks, Super08.