Mine was a 1000 gal butane tank we cut in half and it works pretty well. If I had it to do over again i would make the doors bigger and put sliding racks in it. Early mornin is correct on it being extremely heavy. I wish i had found this site before starting the build and there is alot of things I would have done different. Be sure that you make the fire box big enough, mine is a little undersized and it makes it rough when the weather isn't cooperating. I am going to try insulating it to see if it helps.
There is a link for a pit calculator, I thought I had it but evidently don't, but im sure somebody can give it to you.
Here is mine if it helps.