I recently cut a propane tank in half. What I did was took one of the nozzles off and filled it full of water. After it was full I drilled 4 or 5 1/4" holes that overlapped so I could get a sawsall blade in there and started. Then I cut down both sides and rolled the tank til I eventually cut all the way around. I made sure I was below the water level when I cut so sparks wouldn't be an issue. It probly took me 30 minutes to cut the tank once it was full of water.
As far as burn out goes, I waited to do it til I had my doors cut in and operating so I could get fire all the way in the back of the tank. Naturally it flowed to the open end so I filled it full of good dry hickory and kicked back for an hour or so. For the first maybe 15 minutes or so there was some ugly black smoke pouring out but after that it was good to go. No more propane smell whatsoever. Hope this helps