Thanks for the Info. I didn't think about the ash, I will look into that, Thanks again.
I would put the gap under the water pan at least 1.5" but no more than 2.5". You just need to give yourself enough space for heat and smoke to move around the pan and up through the cooking chamber. Anything more then 2.5"-3" is just wasting space. I would leave 1.5" around the sides. This will allow plenty of heat and smoke space. I had a buddy do a firebox under a propane tank. He did the same setup without the diffuser plate. One thing he did that I really liked was he tacked on the thinnest steel mesh he could find around the openings at the top of the firebox. One thing you get with your firebox directly underneath is ash going into cooking chamber when you throw on new logs. Very Thin steel mesh will prevent that.