I have a turkey fryer I have used one time with a dutch oven found beside a dumpster. Don't know btu output but with a sheet metal shield from ground levet to top of dutch oven it was plenty. Started with wheel weights pile just above top of the rim. As they melted down and I started skimming clips and fluxing. The volume dropped to the point I did not have to worry about lead splashing over the side as I stirred the pot. To hasten the original melt, I poured about a pint of motor oil in the pot as soon as I saw a little melted lead in the bottom. By the time it burned off the weights were mostly melted and I started stirring in pine shavings with a dead cedar limb and got some good clean lead. The frame on the turkey fryer was reinforced so I did not worry about it collapsing.
A bigger pot such as your helium tank will allow more working room as you stir but using too small a load might make it harder to dip out lead as you start to pour molds. Another reason to look for "just the right size".