Difference between 70's and 80's coils.

Sorry don't know. "Coil resistance" is not the only story. Core material, shape of the core, and turns ratio to name "a few"

I recently had a "war" with an offshore ebay seller which ended in stalemate. They market cheap 3rd world junk coils which are SUPPOSED to replace the magneto coils in old Johnson/ Evinrudes. Comparing open circuit secondary output on a tester I have, the junk coils are a FRACTION of the output of old, cracked open OEM coils, and can be seen visually by spark gap.

So there's THAT factor--a so called claimed replacement---that "ain't"