400 Big Block running issues

Hey Jon!

Cover the basics on the ignition system first since you suspect an issues with it. Start at the plugs and work your way back on the system. While you have the plugs out for an inspection, look at the plugs for a rich condition, black soot, or a lean condition, white plug.
Double check wires for cracks. Make sure there not cross firing. Be sure the cap and rotor contacts are clean. Adjust timing. Use as much initial advance as you can get. Until the starter kicks or has a hard time turning when you start it. Then Back it off. Limit total with vacuum line attached to 52*'s.

Then As above, check the carbs float levels. Tune the carb if need be. The miss at a higher rpm leads me to believe your running lean up top in the rpm band. Inspect the fuel filter.

Now some questions for you;

Do you have/know the cam specs or what cam it is?
What size fuel line is on the car?
Does it remain that size to the pick up in the gas tank?

To me, the on and off miss firing could be in the carb. If nothing is found on the ignition.
The carb may need to be opened up and cleaned.