Overheating issue??

[ABX131]

After my maiden voyage in my 63' to test it with brakes, I noticed the truck ran hot, really hot to where it spit coolant out the overflow. It also seperated the brazing on the radiator support (it is using a 57' Chevy radiator with the side mounting brackets). I changed out the thermostat and no difference. We then tore apart and flushed everything looking for any clogged areas. We had solid flow with a garden hose through every port including the heater core. I am confused as to where the issue is....any ideas??