You might want to also find a PROFESSIONAL welder/blacksmith who has experience cutting open propane tanks. Do NOT try this at home! Cutting open a used propane tank is essentially defusing a bomb. Using an acetylene torch to cut open a tank (new or used) has killed more welders than any other welding process.
I am sure there will be people posting "advice" on how to "safely" do this yourself, but they are doing you no favors. Ask yourself this question. Would I try to defuse a bomb with instructions I read from a stranger on the internet?
Despite my warning, I am not trying to discourage you from making a smoker from a propane tank. Let someone who has experience and knows what they are doing, do this process for you. There will be plenty of work to be done afterwards.
Lager,
Juggy