you should get 2 birds on the top rack easily, I don't brine. I try to keep my temp. 400. I also make compound butter of lemon pepper, garlic powder,onion powder and rub it under the skin. should take maybe 1.5-2 hr.
start with all the vents open, close 1 when getting close to target temp nd adjust 2nd one as needed
Why not spatchcock? Try it and you wont go back
18.5 WSM, Weber genesis,little pig grill,crappy smokin pit pro