If you can get a plug for the back of the head, then all you need is a 120Y lower radiator hose.The later A series engines had the heater take off on the pipe going into the engine, so the metal sleeve in the middle of the hose wasn't needed.Makes for a much neater installation.
I've driven (raced) cars in summer with the heater turned on & it doesn't make much difference to the temperature in the cabin, because you're hot already.Usually at that stage I admit defeat & get a larger radiator, usually a 3 core one.