" I had also thought of using a 150gal alluminum fuel tank from a semi-truck for the water tank as well"
Could be it would work, but might have problems. First I geuss you mean for heating the water cause it would probably loose heat really fast if the storage tank. If you put it in direct contact with the fire you will proably ruin it really fast. If you just pass heat around it you might be ok, no flames, no really hot coals near it.
For breaking up large chunks for casting aluminum it is fairly common for us home guys to heat a car wheel or such over a charcoal or wood fire pretty hot then when it cools it is brittle, a good whack with a sledge hammer and we got pieces we can melt for casting
That is a one time heating, not a repeated long term cycle either.
It's good for us to make it brittle that way, not for you of course though. If you have 150gal water it may not heat as well for breaking up, but if in contact with the fire heating it perhaps it will still get brittle at least in spots.
I have several ideas for my heater using a hot water tank. If I have a good gas one out in the yard I am thinking building it up right and use the center hole in the tank as the flue pipe as normal. Just mount it in a 55 barrel standing up, on a little platform, build fire under it, vent through center hole.
Or I may lay it over on it's side welded into the barrel, place a U pipe out of the 55 and back into the tanks center hole. Heat from bottom with fire and center with flue gas.
Either way I would make it easy to get to the center hole for cleaning it out.
A good use for one of those feul tanks might be storage in an area you also need heat.
One might be nice for in my wells pumphouse and a laundry room to keep things from freezing up out there. Build one good fire and get the 150gal aluminum tank full of hot water, then let it radiate out over night to prevent freezing. Insulated small building one tank might be good for days or a week with one fire perhaps?
I was working down at the old storage trailer house a bit for making a work shop out of part of it. I'll need heat later, one of those tanks in an insulated box with no lid under the floor of the work room might work nice also for radiant floor heat. Keep my feet warm. Lots of uses perhaps, wish I could get some freebies on those myself. That's alot of aluminum, the ones I've seen are pretty thick, scrap yards want too much when I do find them there.