It sat so long since it last ran I'll bet the block was dry or almost dry. Since it was not hot the t-stat was closed when you put water in it. Once it got hot enough for the t-stat to open, it dropped cold water into a very hot and dry block. So it flashed into steam creating the geyser. Your t-stat may have been stuck with crud too from drying out that made it stay closed a little longer than normal.
If the block was say 3/4 full of water and 1/4 of air, the block water will just warm up until the t-stat opens and the radiator water replaces the air bubble. The water mixes but there is no flash of steam, you just see the radiator level drop.
If you install a new engine or have one that has dried out, you can fill the radiator with water heated on the stove just below boiling so that it will cause the t-stat to open and fill the block part ways before you even start the engine.
Randy Rosetta KDØKWX TLCA #19351 69 FJ40 "Meanie" & 1970 Boss 302 Mustang