My reverse flow that is trailer mounted has an old water heater tank as the fire box. Its considerable thicker then a drum, much easier to weld on. At least on the outside anyways. The inside was a different story, I had a heck of a time until I figured out what the problem was. The glass lining was dealing me a fit, then I started grounding my welder to the piece I was working on instead of grounding it to the trailer like I always do.
Not sure if I did right or wrong but after I got the tank all welded up I closed the damper between the firebox and the cook area and built one heck of a fire. After that I just started using it like any other smoker and firebox.
If you use a gas heater tank, dont forget that there is a pipe right up the center for the fumes and heat to travel up and out the flu. On mine I cut the bottom all the way around and then went to the top and cut just above where the flange started getting skinny. This let the whole center tube and bottom plate come out as one piece. I wish I had pics but I never took any of that project.