The reason I asked is because there are a few ways you can do this.
I have seen some where they have 2 barrels, one below the other and the bottom one being the Fire box with 2 small pipes coming into the main one. I've seen it where the bottom barrel has been either directly below the top one and I've seen it where the bottom one is set back from the top one and the pipes are angled.
I have seen some where they did a half barrel to one side of the main barrel and offset it so it looks more like a traditional smoker.
The fun part about this is that you can customize it to whichever way you like. Take some time and look around the site, I'm sure there are some pictures here somewhere of some of them. I like the Rotisserie idea.
I don't think there is any benefit to having it either way, just whichever way you like best. Once you learn your smoker, it won't matter how it's set up as you will know how to obtain and keep the temps you are looking for. The one thing I can say about having the fire box either below or below and behind is that the smoker would then come in from the bottom and be more even. When you have a box on the side, then typically the unit is hotter closer to that fire box and the food you place closer to there will cook quicker.
Good Luck, let us know what you decide and of course take Pictures, we LOVE pictures.