If you are concerned about the tank exploding while using heat (torch / grinder, etc) on it, create an air flow. Put compressed air or CO2 (car exhaust will do) line to one side (end) and open vent on the other side (end). As long as enough air is flowing through the tank, it will not explode. it may create a flame out the exhaust side of the air flow, but that is much better than the other options.
Of course flush first as you already suggested.
22.5 WSM, 1997 Performer, 2010 Performer, 1984 Kettle,
hardly ever used Gas Grille