It shouldn't be a problem to fit that much wood in the fire box... most smoking woods are fairly heavy, even if dry. Don't be afraid to let the wood touch the sides and each other. It can even touch the top of the box, though you don't want it lifting the box open. Do use some larger chunks as those will smoke longer than a bunch of small chunks. Otherwise, don't worry about adding more wood as there will be plenty in the firebox to get you through the first few hours, plus smoke will linger in the smoker for a long time. After that you'll have already thoroughly smoked things and the food will be a temp where it won't readily take more smoke anyway.