I am just seeing this again and agree with what Bob S said, racers have been blocking those off for decades with no ill effects and in fact Pontiac itself started blocking the flow to the core in 69 using a vacuum controlled shutoff in cars with a/c. This was done to combat the radiant heat from having a very hot heater core under the dash. Even though the blend doors were shut so the incoming fresh air couldn't push it through the vents it caused the whole heater box under the dash to convect heat fighting the a/c in 7s and 8s. Being in a hot climate you are smart to stop this. Just to revisit your original question, a couple of ways I have thought about stopping the flow to combat this in the summer months and still keep it original looking is one: have a couple of pieces of brass or aluminum turned down on a lathe to just fit inside the hoses and install in the ends of the hoses with an extra clamp around them to make sure they stay in place. or two install 2 shut off valves in the most inconspicuous place on both hoses so even convection heat from the other hose could not get to it. a lot of cars including foreign used these on the inlet hose from the late 60s on. The other option would be pcv ball valves. It's to bad the block offs they offer aren't any good because that's the best non original way to go. Unfortunately in all the first gen birds I have owned and seen through the years I have never seen an original heater delete car. Probably being from the northeast doesn't help.