Is that a stock type iron head with a OEM type spark plug or a 3/4 inch SB type plug with the 5/8 inch socket size?
If stock try a Champion RJ10YC or a RJ12YC or a Autolite #84 or 85.
If it is the smaller 5/8 socket size with the 3/4 inch reach try the Autolite # 23 or 24 up
IHTHs

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