... Also wondering what's wrong with the exhaust pipes. Too much? Seems excessive to me too. Should I build exhaust for the main chamber and then another in the rib box with a closure between the two?
To put it delicately, the exhaust is unconventional.
You have three small diameter pipes. Two exiting at the top of the CC and one exiting at the bottom of the CC.
Maybe it will flow OK, who knows? Give it a try. If it needs to breathe better you can always slice them off and put in a normal stack.
If you add the rib box, those exhausts will be replaced anyway, no matter which end you put the box on.
As for the rib box, am not sure what you are wanting to do. You can put it at the far end, but it may not get hot enough.
If you put it over the firebox, you will need to convert it to reverse flow.
Either way, you will want to have the exhaust go out the top of the added box with an opening between the CC and the box in order to cook in the box with smoke.
Take a look at other builds with rib boxes to get an idea what you are up against. Be advised that there is a difference between a rib box for added cooking area, and warming boxes that are not intended to get up to cooking temps.
Also take measurements of the cook chamber and fire box and plug them into one of the pit ratio calculators to see where you might have to make adjustments.
I hope this helps.