@wrkinprogress The porsche 914 had a similar setup with a fan in the engine bay that sucked air in and pushed it through the heat exchangers then through a damper on each side into the cabin to be distributed at the dashboard. I don't remember having to have air going through the heat exchangers when no-heat was selected. I'm sure you could devise a way for a type I motor. The 914 was a type IV.
Inside the original type I heat exchangers there is a heat sink that surrounds the J pipe. This is critical in making heat. I think some aftermarket heat exchangers don't have this, or it's not as efficient.
One more thing. If you have a bigger than stock engine and you want to maintain some performance, the stock exhaust with heat exchangers usually have a small diameter exhaust pipe. They're might be an aftermarket set that has a slightly bigger pipe. I'm not sure.
I have a Spyder with a 2276 Type I. I've played around with an idea to fabricate my own heat exchangers with the 914 fan setup. This is one of many thoughts bouncing around my head that'll never come to pass though. I should just pony up and get a heater like Gordon and Danny's. Danny has a Spyder and his heater is under the dashboard.