I'd recommend 275 cook temp, some will say even higher.
You can probe with a toothpick, or, I prefer the "bend test". Pick the rack up with tongs. If they bend in 90* angle, you're good.
Full racks of spare are usually about 5 hours for me.
Good luck and don't forget the pron!
Smokey Joe Silver
Stealthy super fast grey thermapen