Champion RN9YC(also RC9YCwill work) or maybe, if you can find them ,one step hotter RN10YCand RC10YC or Autolite #3924 or 3925, those two Autolite plugs have the 5/8 inch hex on them but they will work fine in your motor. I can't remember the proper part # for the standard Autolite 3/4 inch reach resistor spark plugs with the standard 13/16 hex on them blush

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