some people recommend not trussing the legs. It allows the dark meat to cook more evenly with the breast meat. Similar to icing the breast, but without the ice. When doing an untrussed bird at higher temps, it helps to make sure there is air flow through the center of bird so it cooks from the inside too. If you fill it up with so much stuff that it impedes the flow, the cook will end up closer to what you describe.
Nothing wrong with cooking at 350 if you like good skin, but it's possible the grate temps were hotter than the dome temp..
I personally prefer my dark meat cooked to 180+. By trussing the bird, you are slowing down the cooking process of the dark parts. By the time dark is in the right zone, white is overcooked when trussed or stuffed.
You might try spatchcocking or splitting?
Modded Brinkmann Gourmet. Basic UDS. Rescued 22.5" OTS. SJS Mini-WSM. Stubbs. B&B Oak Lump.
Cannondale 2009 F8 (for burning off all the great BBQ)