I just installed one in my 67 M35 a few weeks ago. Rather than trying to mount it up in the engine compartment like the instructions were trying to tell me to do, and running the ducting through the firewall, and losing precious heat, I just mounted mine under the dash on the passenger side. I had seen that other guys had done it that way, and that's how the one in my 66 M35 is installed. I think it's much better, because that way you are recirculating the warm air, rather than constantly sucking in new cold air too.
I ran out of time and did not manage to run the ducting up to the windshield defrosters yet, but the heater works so well with the warm air just blowing out under the dash, that it hasn't been a problem yet. I also did not take the time to wire up the resistor to where the switch gives you high and low speeds. So mine is either on or off right now. Not enough time lately to do things thoroughly.
Since I decided to mount mine where I did, of course the engine water hoses that came with the kit were not long enough, so I had to go to the auto parts store and get some longer ones. Even though my heater kit came with instructions, they were not very helpful, and did not tell me where I needed to attach the hoses to the engine to get the water to flow through the heater core. So that took some trial and error figuring out where I had pressure around the water pump. Wish it wasn't dark out now, or I would go out and get some pictures to include in this post. Maybe I'll ad some tomorrow.