I guess it can be done, but you better d--ned sure know what you're doing or you'll be the first and last thing cooked on that cooker.
Mike

I saw a man use a flexable pipe ran it from exhaust of his Portable welder to the propane tank. Somehow the Carbon monoxide disapated the propane gas fumes. Had no problem then cutting ends out of the tank.

I've cut A few up for various reasons over the years but I like to wash them out with Dawn dish soap and let them sit open to the atmosphere for a few months before I cut them.
I like to use a Sawsall to cut them as I have less grinding to do and the chance of sparks is greatly reduced. If A torch is used A few pounds of dry ice put into the tank will drive out any oxygen. Put the dry ice in the tank and leave it in for A few hours (this gives you a chance to check on your health and life insurance) I don't like the exhaust gas trick as if your vehicle is running rich you might be putting gasoline vapor into the tank and this would not be good. At the vary least fill the tank to the cut line with water before you cut it.
The best bet is to use a new tank from Tractor Supply or the like. I don't think they cost that much (50 bucks?) and it's A damn site safer!

Good Luck!
H.R.
PS: This is only my experence,Your results may vary.