Congrats. The big hurdle is now over. The thermostat is normally close. So unless, the water temp goes high and it only takes about a minute or less, then it will slowly open. If it continues to stay close, remove the thermostat for now. Do you have a tell tale on the thermostat housing that you can tell water is not reaching the thermostat? Remember, this motor is on a bench and the water impeller is not connected. So you will need to increase pressure on the water hose so the water can go to the cooling ports.
If everything fails, remover the t-stat housing and t-stat. With engine off, shove a hose and keep a seal using rugs and hands, to the t-stat housing and turn on the water slowly. Just make sure you put a cover on all the electrical parts. The water should drain to the exhaust bellow. If not then you have a blockage.