Magnum heads...what spark plug?

Magnum heads (EQ's included) only use a 3/4 reach plug. I have a set on my 408 and am using NGK BKR6E. My engine is 9.8 to 1 compression. Depending on your compression ratio you may need to go up to a higher #. With NGK the higher the # the colder the plug

BTW: I hope you have adequate piston to head clearance. Early 340's had pistons that stuck up out of the hole and EQ's are closed chamber heads.