New LS3 start up but no water flow through radiator

[vstol]

Finally got my LS3 427 started but the water is not flowing through the block. All parts are new to include the thermostat and a Dewitt radiator. Temp rises but radiator is cold. This is the first few starts, so I only run it for a minute or so since the engine temp rises. First thing I think of is the thermostat but I remember seeing posts about getting the water to initially flow. thanks