Get a bloke in
If you are intending to use an old pressure vessel or fuel container for a project such as this, or are at all uncertain of your own ability to do this part of the project, take it to a company that can do it for you. They will have access to inert gas to flood the container and remove any oxygen before cutting or drilling into it, so that it no longer presents a hazard.
Once the container has been opened to the extent that it can no longer confine the explosive gases that can build up during the cutting process, it is safe to work on to complete your project. The opening must be sizeable — for example, just removing the valve on an LPG bottle is not enough. The rate of gas expansion if the vapour combusts will exceed the opening’s ability to dissipate it, with potentially disastrous consequences.