Will have some pics next week. I have the firebox monted on the botton with a piece of 6" pipe from the firebox to the main cooking chamber. The firebox is 16" across by 40" long. The chimney will be 4" x 32" long. Will be able to use charcoal or logs. I think I see what you are saying. The pipe would not have the surface area of the plate and would not cover the whole bottom of the cooker to have even heat. Can't get to the cooker untill Tues. The shop is closed untill then.
Here is the website of how I got the chimeny lenght
Hope it helps someone.