ideas for pit design

I am getting ready to work on the inside of the pit i am making, and curious to get thoughts on what would be a good way to set it up. i am thinking of putting 3 shelves inside, about 9 inches apart, the top shelf about half the depth due to tank. the shelves are going to be removable, and i am planning on putting a rotisserie on it, with the ability to run center for whole hog, or 4 thin shelves for standard configuration. i am planning on putting in a propane burner system in the bottom of it so i can if need be convert it into a grill for cooking burgers / brauts for local fund raisers. the firebox is fed in via a 6" stove pipe from the bottom / rear of the smoker, and two - 3" vents with dampers. i am trying to decide if i should make larger vents or the two 3" would work.

any thoughts or advise on how i might make it differently, or suggestions? it has been a long time in the making and i want to make sure that it is set up properly.

here is a shot of the firebox into the pit with the 6" pipe in. the two vents out are kind of able to be seen - two 3 inch vents one on each side. i am not sure if i need to make them larger, or keep them at the 3 inch. i am going to be putting dampers in each of the tow out vent pipes as well. and capping them with anti-rain caps

the firebox is an old sears home heater, that now has a new purpose in its life. i have been thinking about later on if need be getting one of the remote temp sensor units that has a blower and can be monitored remotely, and put the blower in the elbow of the in pipe.

i have several shelves from a commercial deep fryer stainless steel, with holes all over the steel, with two handles that i plan to be able to hook into the main shelves and hang down for additional shelving, and can be easily removed depending on what is being cooked. i am going to also try to use them for shelves for the rotisserie