I responded to a post similar to this only to discover later it was an off set fire box type. If your upright is the square smoke n pit with a water bowl and a charcoal bowl, and only if, the instructions below might help you as they did me....
I have two Smoke N Pits, a new one and one from the late 80's. In the new one, the only way to hit those temps are as follows...
#1. In the bottom of the box, using a uni-bit, drill one hole on each side about the size of a quarter. When you are not smoking at high temps, these holes can be covered with fridge magnets or dollar coins. The holes will draw in additional air, and can be used as drain holes when hosing out your smoker.
#2. Drill about 8 1/2" holes on opposite sides of the charcoal bowl. You'll end up with a total of 16 holes, 8 on each side. Space the holes and include some near the bottom of the pan.
#3. Purchase a small sheet of expansion metal, light gauge will do. Take a paper plate and using a black marker, draw a circle about 1/2" larger then the plate. Use the plate for a guide. We have to use something to support the metal mesh, I used the circular ring off of a beer can chicken holder. Anything metal will do, just so long as it is clean metal. Place the metal ring on bottom of pan, and place mesh on top. Now air will flow freely through your charcoal pan. A few taps on the side and ash will drop below the mesh we inserted.
Using these mods and LUMP COAL, I was able to hit over 400 degrees reliably and consistently. Just make sure to cover those two holes in bottom on smoker when you are low temp smoking.