spares or backs? both can take that heat, spares take longer so it would alter the timing some.
I usually ramp up to 300-350 when finishing ribs. I'm thinking if you don't want to do them that high, you could start the ribs (spares) several hours prior, slooowly ramp up to your chicken temp and drop the thighs in when the ribs are done. Hold them foiled in a cooler and put them back on when the thighs are good. sauce them all and let it set up in the high heat.
lets see if I can give you a better idea how I would approach this if I was doing spares.
time: 0, temp: 150 spares on
time: 1 hr, temp: 250
time: 2 hr, temp: 300
time: 6 hr, temp: 350 spare off (foiled, coolered), thighs on
time: 7 hr, temp 350 spares on, sauced, thighs sauced
time: 7:15 all off
for backs, hit 350 after 3 hours and get the chicken on then.
remember, its all by feel, I'm just guessing how long the ribs and thighs will take.