totally lost

My smoker is only a couple days away from being done and I think is similar to what you are looking to do. The firebox is going to be mounted on the back of the tank directly in the center with a large plate over the entry. 

http://www.pigpitbarbecue.com/blog/2014/8/8/new-smoker-progress

Hey FatRat.... it sucks when life gets in the way of smoking but it happens to us all. You're smoker will get there. Yes... I've read several posts here as well as several google searches. My tank was used for liquid fertilizer. It turns out that's easier to rid of residue than gas or oil. It's not explosive or flammable but the fumes it has can be. So, a water flush should solve that. Still taking it to a pro though to get me started.

Football is awesome! Best of luck to your son.