Running Lean?
Spread bore carbs do run a wider jet spread than a square bore. What is the list # on your carb? I remember a 6210 being one of the 650 spread bore Holley's and I just looked in my Holley book and see it came with 60's in the primary side and 83's in the sec side so it sounds like yours might be jetted richer than normal but IMO your plugs don't look too bad, just a little rich at idle. Not having enough initial timing can also have an effect on that as well as the adjustments. You do need to get the timing dialed in before you worry about carb adjustments. If it's mainly a street driven car I'd suggest running the vacuum advance. It'll help the plugs run cleaner as well as help with fuel mileage and have peppier off idle response.
TB is right about the wideband O2 sensor. I love mine. You can dial in every circuit for optimum performance