Colder plugs are really only needed if flogging the engine long and hard in hot weather. D8's will make the bike so cold blooded you can hardly ride it until completely warmed up. There is no need for colder heat range on the stock motor. I would think extended nose plugs might run some risk of hitting the piston crown.
You do want some resistance in the spark plug circuit. Resistance wire, plug caps or plugs. The stock plug caps for the 74 CB550 were 10K ohms, with 5k showing up sometimes. The 750 used 5K plug caps, slightly different coils.
You can measure the plugs for resistance with an ohm meter. Measure from center electrode tip to where the wire attaches.
I thought the Dyna wires were suppression wires with internal resistance. The last ones I saw were, anyway. If so, you don't want resistor plug caps or resistor plugs using those. If your wires have metal core, you want either resistor plugs or plug caps, not both. Some resistance in circuit will improve the spark and extend spark plug life. It will spark if there is resistance or not, though.
You didn't mention much about your coils. If they have 3 ohm primaries, you will probably have battery charging issues, as they draw more power than the stock 5 ohm coils do. At idle, the battery will discharge faster. When above 2500 or so, the battery will recharge slower. When the battery voltage goes low enough, no more spark!
It helps if you don't run any lighting. The 550 charging system is only 150 watts and then only at around 5000 RPM.
