Mike, welcome and please read this carefully.
Many many people have made great smokers out of used buel drums. I've been reading these bbq forums for about 5 years now, and I've just heard too many stories about people trying to cut open those drums that weren't properly bled of fuel, and the result has been death and some horrible disfiguration. One guy got blown threw the roof of his welding shop, with half his face missing.
Please, first thing, make sure an expert bleeds the tank before he takes a torch to it.
That said, I think you'll be fine with a good design. One real good idea is to look at pits at www.gatorpit.net
for great ideas to steal!
(sorry Rich, I'd only reccomend that he steal from the best!)