I don't think its a flow issue. The time it would take for the stationary water in the block to convectivly heat the water in the radiator to the boiling point would damage the motor.
Plus to verify flow all you have to do is feel the radiator hoses as you watch the gauge.
Since the only thing that appears to have changed is the radiator to look elsewhere seems pointless. Is the radiator the proper number of rows and the same flow pattern (top to bottom) as the old?
