Help with cooking chicken?

I am baking a chicken and stuffing cerole and the recipe said cook chicken halves for 30 minutes at 400 degrees. well i cooked whole boneless skinless chicken ..so how long should it take? i mean there is other stuff in there too so i dont want to just leave it in there for an hour..it might mess up the stuffing ..i dont know. please help. i thought maybe i would add another 20 minutes and bring the heat down to 300...sound good? please help!