Many fork lift tanks are aluminum,but if its steel....Open the valve and bleed off the contents.Remove the valve(s) and fill with water.Rinse out and drain.The propane is now gone.If you want to be super safe,drill several 1/2" holes in the area to be cut off,to provide venting.Cut with a torch or carbide cut off blade.Scrap yards are a good source for all kinds of heavy pipe of large diameter.