Do you get any water out through the side drain in the block? Real common place to be plugged, usually can be cleaned out with wire. My favorite method of backflushing these is to pull the heater hoses at the heater, plug the lower and put a hose barb in the upper. Take out the thermostat and put the cover back on. Remove the block drain cock and leave the radiator cap on and the radiator drain cock closed, then slowly turn on the water. If nothing comes out, start rooting around with wire until you get sprayed in the face.
Then its time to remove the lower radiator hose and complete the flush.
FWIW, the latest Vintage Truck mag has an article on antique radiators. Writer recommends against using commercial stop-leaks, says to use a handfull of ground black pepper.
