Spark advance should always be tailored to the compression ratio of an engine and speed it runs. Normally you can do comparison to different models like you said , newer models, and come up with satisfactory performance. A 35 or 40 degree advance was for a very low comp engine and when you increase bore size that changes comp ratio as you are squeezing more air into same size comb chamber, then you put in dome pistons further increasing it. Not a one size fits all. IH actually made a special impulse coupler for the H4 magneto to accomadate fire crater pistons on the M and H. Put one in for a guy who would not go to battery ignitions, about 45-50 years ago.
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