Mike,
Dry sooty plugs are a typical sign of over fueling. With the HEI you should be able to fire through just about any rich mixture.
If you look at the list of things you have addressed, it covers all the suspects. I think this is why gtow69 recommended the scope check...you have already done the things most mechanics would have done and still have the problem.
The carburetor sure seems like a prime suspect to me. It is one of the few items that hasn't been swapped and the plugs are pointing to it.
If you have the ability, I would check the float level first. I would also dig around some more with the carb cleaner focusing on the intake to head area.
When you installed the HEI did you dump any resistor in the old ignition circuit? Is there a vacuum hose to the tranny modulator...is it in good shape?
You said it 'runs rough' but I wonder if you don't mean it runs fine but 'idles rough'. My bet is it runs great but just idles funky. You are getting close, don't give up!
Stuart
