"Mounting the gravity fed smoker on a curved surface looked like it would be a challenge, perhaps you could think of a simple solution for mounting it on the drums curved surface? "
The outlet is 3 1/2", I can add a flange on outlet.
Also I can add 3 legs on the burner, the height of legs can be adjusted to match USD.
You need only to drill a hole of 3 1/2 +" on your USD.
Let the USD and burner stand side by side.
Legs for your reference