Just finished cleaning up the cut with an angle grinder on my old helium tank. It is pretty heavy gage steel and is 9.5" wide and 7" deep. Now per the info above I might go pour some water into it and measure it's volume.
Edit; Just poured water and 1gal is 3.5" below the top and 1.5gal is 2.25" below the top! So this should be more than adequate for what I want to do.