I put 2 inline ball valves from Home Depot in my 79. It worked so well I can't believe that I didn't do this 15 yrs ago. I plan on by passing the heater core on my 73 in a week or two because now that its on the road I can feel the cabin heat even with the top down.
I have found it's better to block both hoses. One thing I like about this is if I need heat(not that I ever drive it in the winter) all I have to do is open up the valves and in the mean time no more burnt feet.
I also like the suggestion that is directly above mine. If the fitting would come out of my water pump and intake on my 73 I would just put in pipe plugs. I bet I would brake them since they have been in there for 39 years.