Take the return line off at the manifold and w your rubber tipped air nozzle blow the coolant in that foot or 2 of heater hose in reverse thru the heater core. Use your hand to modulate the psi as water faucet psi is ~60 psi (at least in Nebraska) and heater core/rad are designed for ~15-20 max. If you think the rad is partially plugged best would be to take it off and back flush it the same or take it to a rad shop & let them boil it or even "rod it out" if it's really plugged w scale as chemical flushes are not stout enough to attack scale w much effectiveness. Rodding out can be dangerous to tubes that are old and your rad shop can further advise you. Back flushing is way less effective on the block/intake cause the passages are so much larger/unrestricted but if enough debris builds up in the bottom of the block cyl cyl cooling (hot spots that produce steam) will be affected but this requires core plug removal but is worth the effort. Dont know how far you want to go on your deal but throwing stuff out there