When I do BB ribs in my #2, I let them go to 5-5.5 hours ... 6 hours may have been too long, but then again it also depends on the amount of meat on the bone. I also set my temp to 225-235F for a 5 to 5.5 hour smoke with about 4oz of wood (typically hickory). I take my ribs out at about 4.5 hours and apply a dose of BBQ sauce to the ribs, but this is just my preference, it is not a necessary step. I also put a small loaf pan of apple juice in the bottom of the SI next the smoke box just for added moisture. You can't really go by the IT of the ribs since it would be tough to get a good reading with all of the bones.
For your next try, I would suggest the small loaf pan of juice, set the temp for 225 to 235F, and check your ribs at the 5 hour mark. If the meat looks like it is pulling back from the bone, then you are pretty much done. If not, give them another 30 minutes. You can also try a toothpick that should easy go in and pull out from the meat.