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 IHTHs
NGK R5671-7 crosses to an Autolite 4054. Autolite does a 4055 and 4056 - 1 and 2 steps hotter. Standard non-resistor, 3/4 reach, 13/16 hex. 4056 is equivalent heat to the old 64.
Not to get off topic, but is the a site that a guy can go to that shows cross reference on multiple plug manufacturers? I looked the other day and had a real hard time crossing some. Jim
NGK does make a #6 plug..........Like asked, cyl. head makes all the difference but here's a number that looks like it should work for an aftermarket head....R5674-6 or 4449 stocking number..
I don't think you can use a 3/4 reach spark plug with the stock 13/16 plug socket size, I'm almost positive you need to us the smaller 5/8 socket size plugs Champion plug numbers RN9YC versus a RC9YC. Autolite resistor plug # in 5/8 size I use are 3921 and up to 3924 in my race gas motors and the same 5/8 size in non resistor # in retracted gap are 3931 to 3935, 3931 is very cold.