Nice looking tank dpeatrowsky
. As far as the fire box I suggest using Feldon's calculator to determine the size of your box, stack, and everything else you will need for you new build. Heres the link. http://www.feldoncentral.com/bbqcalculator.html
My box is 28.5" wide, 20.5" tall and 24" tall. Are you building a Reverse Flow or straight flow?