275gallon smoker build

So I was given a pair of 275 gallon take for building a smoker. I'm wonder how big of a firebox I should for it. I'm thinking of welding them together in to a build box or use the second for the metal to build the firebox and sealing the doors. Any suggestions ? I'm do know I want reverse flow and use the reverse flow pan for a water tank for moisture. Once I get this one built and working great it will be time for upping to a pair of 500 gallon lp tanks in a few years.