My biggest concern would be if the tank was at some point used for leaded gasoline, I would not use it for a water tank for livestock.
Steel tanks are heavy, hard to manhandle, and they corrode depending on your water quality. The small fitting on a fuel tank will make you cuss how long it takes to empty the tank. :bang:
If the tank is still usable as a fuel tank, I would suggest you put an ad in a trader paper or take the tank to a consignment auction and you might get $100 - $150 to use toward purchase of a poly water tank with a 2" drain valve and hose designed for the purpose.
When it comes to jobs that get real old real fast, IMO hauling water is at the very top of the list! I did that for 3 summers in the peak of our last drought. Had a 1500 gal tank in my old 1961 Chevy truck. Sometimes made 3 trips a day to get water from a potable water system, 34 miles round trip. Needless to say, did not get much else done. Water cost wasn't so bad. Fuel and time was another thing. Never want to do it again, and am set up now so don't have to.
Don't know what kind of rig you're planning to haul with or distance involved. A good HD 3/4 or 1 ton pickup can easily handle a 400 -450 gal tank. Half ton probably should go with 250 - 300. I also had a 500 gal poly tank that worked good on my flatbed trailer. Neighbor had a 1200 but it was a little heavy for his trailer.
Welcome to CT! :banana: