So, why is it that the original Marelli coils of that era did just fine at the proper 3 ohm resistance, while Dynatek recommends 5 ohms type coils?
It's likely that the oil filled, metal housed originals dissipate heat better then the solid epoxy potted Dynatek coils.
One of my 3-Ohm Dyna coils cracked open some years ago, and now I have the 5-ohms type, but they actually measure at 5.8 OHms. What is the optimum primary resistance?
It's a tradeoff. Lower resistance can give a hotter spark (depends on coil design), more current consumption, and depending on the system design, possibly more points wear. Higher resistance gives lower current consumption, lower coil temperature, and again, depending on design, possibly weaker spark.
When I ran Dynatek 3Ω coils in my Mille, I didn't like their high temperature and the reports of failures, so I put 1Ω resistors in series with their primaries. It sounds strange, but that reduces coil heating to slightly more than half the original amount, with little change in spark intensity.