Hey Dave, the only thing that comes to mind is that you are adding wood too long into the process. No smoke from wood for more than 3 hours, and you should not have any bitterness. I use primarily hickory, and have never had a bitter piece of meat. Let the lump charcoal continue to provide the heat after 3 hours. I use the entire piece of wood as well, even the attached bark. I won't add just bark, but if it's on the log, it goes in. I use only dry wood, no green wood, and I do not soak it.
I use the Minion Method. I prep a bed of lump, then a layer of wood, then a layer of lump, then a layer of wood, then I dump my coals from the chimney starter on top, and add a few chunks of wood. This will get my smoker to 270-300 in about 15 minutes, then I add my meat and some more wood.
I hope this helps, but the box looks awesome by the way. Looks like a row of nickels where the welds are.