There are a couple of different coils used on the 390 platform, Jimbo.
Of the oem variety, the polarity is reversed on at least two of them.
The coils also have a built in advance that changes (similar to the pvl coils you're familiar with) above a certain rpm (2000 if I remember correctly.)
The air gap of the stock coil doesn't seem to take away much timing at all. We've opened them waaaay up like is popular on the clone stuff now, and really didn't see much (very small retard) change using a timing light.
If you're able to use an aftermarket flywheel, then by all means do so...the flywheels on these engines are way overkill, even for the heavy stock rotating assembly.
Hope that helps.
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