What does Eddy say about he correct spark plug length? Follow thier directions
If they say to use a standard 426 Gen 2 spark plug type your golden with that plug as long as it is 3/4 reach I like and use the smaller 5/8 hex spark plugs now instead of the larger 13/16 OEM sizes

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)