I would flush the engine out as much as possible, then get your radiator checked out (or rodded) or borrow a known working one.
To really clear the blockage (if in engine) would be to pull it down. Would first check out all the main water bits such as smaller bypass pipes (ones on side of block) and make sure they are clear, make sure the water pump is ok and clear etc. Once they are out of the question change radiator as said before and then test it out. If still no good then you need to dig deeper and start pulling the engine appart (head off for a start).
Water flow in radiator is top to bottom.