I'm wondering why everyone doesn't go to a local plumber a get a 40-50 gallon water heater(s), used, for free.
Plumbers regularly pull water heaters for folks and replace them. Some have many years of life left in them, particularly if you replace the sacrificial anode (cheap $).
The electric heaters would be the best, no center tunnel to close up.
Lay it on it's side in a steel cradle, use the existing pipe/drain connections to fill/drain/vent.
Or is this old news?