I have built 6 smokers from propane tanks of 125-250 gallons using 100# propane cylinders as fire boxes. I filled each one with water and poured it out and started cutting with an acetylene torch immediately afterwards. I have not had any problems using this method.
I would have been terrified of grinding the collar off a propane cylinder with gas inside!
I know this is late in coming, but after you fill the tank with water you can immediately pour out the water and start cutting. the water displaces any remaining gas in the cylinder making it safe to cut with whatever means you have on hand.
You could cut a piece off a brass supply tube and use it to stabilize the wire until you get the hole started.
How do you clean it out between broods and after breeding season?