Gas lines are like electric wires.  Underground prevents a lot of damage.  Absolute minimum distance is 10 feet and I strongly encourage farther.  NH3 tanks are probably better built than LP tanks.   They have different fill, service, and relief valves.   Was yours an applicator tank or a nurse tank?   All the fittings on a propane tank are brass.  It doesn't spark.  If yours was properly converted years ago there is no reason not to use it.  Current law prohibits converting a tank under 1000 gallons.  Some misinterpret that to mean any former NH3 tank.  Old conversions are grandfathered in.