I'm only worried about the inside.
In that case you can stop looking for a tank to put it in.
Get it blocked into a stable position (remembering its going to be HEAVY when full of water)
Tape/insulate the opening you are going to use. This might be the fill pipe opening.
Cut a piece of rebar to maximize how much is inside the tank. (Diagonally from the fill spout?)
Insulate the tip of the rebar so you can have it contact the bottom/side of the tanks when the electrolysis is running without shorting out
Remember which is anode and which is cathode when you hook up the power source!
Plan on a long run. as you have a lot of surface area, and not a lot of anode surface. you may need to add water.
Clean the anode several times as you go.
After you are done, dump the water out and dump in a quart of more of alcohol, and slosh thoroughly. (This is to remove the water)
Dump out the Alcohol and get a quart or more of kerosene in there and slosh thoroughly (This step is to "oil" the surfaces with something that won't contaminate the fuel too badly)
Seal it completely until you are ready to install and fill with fuel (Completely)
Best to fill asap after de-rusting the tank.