I guess in many ways I'm ignorant on things. But I can't see changing from one type spark plug to another giving a "significant" boost in either power or mileage.

I can see where the HEI ignition would / could give a hotter spark, but on these old stovebolt engines that are stock I can't see where that is a great advantage except for reliability. But if the points are properly maintained, you shouldn't have reliability problems anyway.

Perhaps there are some videos out there floating around that show the old style spark plugs firing vs a newer "better" plug. It would have to be slowed down considerably to be able to see the differences (if any) of the firing capabilities of the two types of plugs.

But in fire academy we learned about what is called "Limits of Flamability". If the fuel to air ratio is too lean, it won't burn (or at least not very well) and likely wouldn't run an engine, if the mixture is too rich, it won't fire very well either. And with gasoline the air fuel mixture is pretty narrow window on what will burn correctly and what won't. Whether is is delivered by carb, or injector that mixture can only be so lean before it either won't run, or won't run very well.

Claims of 30 + mpg with a 216 or 235 just seem far fetched to me. My VW Passat does well to get 30+ mpg and it's 4 cylinders in a car that's a fair amount lighter than these stovebolts.

I can see a good tune up bringing the mileage up a little, but I just can't see 25% better just by changing plugs to a different style. But again, I'm fairly ignorant on these things. Maybe someone knows a secret I don't.