It's in an oven? I was thinking open fire/coals. That's different then. I would cook it like a turkey. Maybe stop the thing and check the temp with a meat thermometer at 3 hours after cooking at 275. 250 even better. Go for an interior temp deep in the shoulder of about 195, and cook until you get it there. After cooking it up to that, turn the oven off and let it rest in the warmth for another hour. Just thoughts. I could be wrong!
Big homemade smoker on a trailer, Weber grill, firepit