Are you making sure that the block is full of coolant first, then adding the rad?
Take the thermostat out and fill the block from there, then put it back in and top off the radiator. Put the cap back on and fill the res tank, then start the car and run it till its warm, open the bleeder on the thermostat housing for a few seconds and close it. Do that atleast 5-10 times as its heating up to get rid of all the air in the system.
If that doesnt help anything, you likely do have a real blockage of coolant flow and not just a massive amount of air in the system.
