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?
As you can see from the pics, mine is a Reverse Flow. If you're not too far along with your build, I would suggest you take a look at going with Reverse flow. I currently own both and wouldn't ever go with straight flow again. I get much better temperature control with the reverse flow.
What ever you decide, use the Feldon calculator as your guide. I also suggest checking out http://www.smokerbuilder.com
I hear ya about the cost of sheet steel. You got to do what you can afford to do.
Good luck with the build and feel free to fire any questions at me that you may have.