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A spark plug cross reference chart that appeared on this site should match the Autolite 353 plug to the champion J6J, not J4J. The last number in Autolite is heat range, which hasn't changed from their old number,A3X ( A = 14 MM, 3 = heat range, X = clipped grounding electrode). I double checked with my gauge, a piece of .020 safety wire, and its reach into a J6J was dead equal to the 353. Remember from the old outboard manuals: Champion J6J = Autolite A3X = AC M44C. Also matching are Bosch W7E and Prestolite 14 3 X. Look for these plugs at garage sales and swap meets, which is where i find nos vintage plugs. There are still some U.S. made autolite plugs remaining at mom and pop auto stores ( Autolite, too, recently outsourced). I find the plugs mentioned are better at handling what we put them through.
bill
P.S. What are those two small holes for that are drilled in some of the Mac exhaust stacks?

If it's an Azusa exhaust, it's for the "J' bolts that hold the muffler section on. If the holes are round, and not elongated to a length about three times the diameter, it's not an Azusa.

They sold the "cap" seperate so people may have drilled their own holes.

I've been using Champion L78, L4J, and early AC M42 FF plugs in my PP ah58's, for which i've been making the two fin hi compression flatheads. These plugs match each other perfectly, and i may have found out why they last so much longer than the current ones. A spark plug guru in Indiana pointed out that the manufacturers stopped glazing the center electrode insulator around 1974-1975, which before this time was less prone to soaking up the fuel- oil mixture. I don't think this was a sales pitch, because i've bought from and traded plugs with him many times over the years, and he's always seemed sincere.
bill
P.S. Please see my "wanted" ( Go Power papers) in the for sale/ wanted section.