What is the Dia of those exhaust Tubes? Did you square the radius and multiply by Pi (3.14)? Assuming 3" pipe, both stacks would be a little over 7" and one stack 3 1/2" so you are definately in the ball park of horse shoes and hand grenades, which seems to be about as close as it takes to work
My Weber lid has 4-1 inch holes or roughly 3.14 square inches
You are also onto something with the basket size, I learned the hard way on the 2 units I built this year, My drum basket wound up about 16" dia x 5" deep and it runs rock solid.
Your smoker is the first Ive seen using a gate valve on the inlet, let me know how it works for you.
"Assumption is the Mother of all F___ups!"