Yeah I saw that post. Very nicely done.
I may install an air heater, since the unit its self tends to be cheaper and more common.
But I would like to use a hydronic heater if possible.
The issue I'd see with that would be the exhaust, I'd imagine if it were anywhere upstream of the cabin there would be the possibility of fumes getting into the car, unless you ran a long exhaust pipe all the way to the back, which could be challenging. It would be nice to tie the heat into the existing ductwork but again would be challenging. No I personally think the best and easiest solution is to just add heat to the back of the car for cabin heat and then use the existing HVAC to keep the windscreen clear and maybe supply a little heat to your feet. I wonder how they distributed heat to the old VW Bugs that had the Stewert Warner gas heaters and where they put the heater.....from what I remember the cabin heat came from the back floor area, not sure if/how they got it up to the windscreen....