When you say tough and dry, was the meat crispy or meaty? If it was crispy, they were over cooked (I know that wasn't the case). If it was still meaty, they were undercooked. They tasted dry with a tough bite because the collagen connective tissue did not melt so they needed to go longer. It is counterintuitive but that's the reality with tough cuts of meat. Use the search feature to learn the probe and bend test.
Personally I smoke a 3 lb rack of SLC trimmed spare ribs at 250F for 6 hours, no wrapping, and they can go a little longer. Untrimmed, the rack will weigh 5.5 to 7.5 lbs. I crank the temp up to 275F for 6 to 6.5 hours, no wrapping. The two bones on the thin end get a little crunchy, but I kind of like it. The rest is perfect.