One thing to remember about any of these aftermarket kits -- they don't introduce fresh air into the system. They only recirculate the air in the cab.
If your car is solely a summer driver -- like 90% of classics -- then it really doesn't matter. But if you ever drive the car in mixed weather where you use the defog, be very, very careful. You'll be driving along at 60 mph, have your window start to fog up, hit the defog setting, and all of a sudden the windshield completely fogs over, making you blind to the road. This lasts for about 30 seconds until the dehumidified air catches up and clears the window. After scrambling for three of four seconds, I finally rolled down the window and stuck my head out so that I wouldn't crash. I was lucky that the road was relatively straight.
When I called tech support to mention the glitch in their system, they said "oh yeah" and pointed out the fresh air vs. recirculated aspect of their system. Their solution? Roll the window down. That's how you reintroduce fresh air. You spend a ton of money and they tell you to roll the window down. Precious.
Now when I drive, I never use the compressor-activated defog. I just use the heater's air channeled through the defog vents and keep the side window cracked. It works well.