Firebox size and tuning plates

[jeepmanlan]

Hello!  I just wanted to ask another opinion.  I have 120 gallon cook chamber approximately 24 x 65.  I cut the firebox to cook chamber opening 5 3/4" up from the bottom.  I want to know if this is correct.  The firebox will be 24" wide x 20" tall and 20" long.  Will this be big enough?  I can make CC opening bigger if need be.  Also, I would like to add tuning plates.  Would 4" from the bottom of CC then 4" to food rack work?  Thanks!!