I agree---the stock heater should be adequate (even at 20 below---it gets COLD in Sweden) You should look at the noisy fan (almost as bad as the one in a 240, to get to) as you need to run the fan to get decent heat with the limited flow-through of these older cars, and make sure the core is not obstructed with debris. I had one once that was absolutely packed solid with winged Maple tree seeds. It was so bad that they physically obstructed the fan from turning. Result--no heat. It took hours to pick the seeds out from in between the heater core fins with tweezers. That got the heat working, but it was still a little less than I would have liked on the coldest mornings. A hotter thermostat made all the difference in the world. If necessary, you can block off part of the radiator with cardboard or thin sheet metal for a similar effect (just remember to take the baffle out when the weather warms up) PVs and 122s had a radiator blind behind the grill that could be operated by pulling a knob inside the car. For whatever reason, 140s, (and all later models) no longer used this feature.

If you still want to convert the A/C unit to a heater, the best way would be to install a new brass heater core the same size as the evaporator in its place. How to plumb it you will have to figure out for yourself. There is some chance that the plastic of the evaporator housing is not suitable for high heat, and will warp when subjected to it, so it might turn out not to be a good idea. On every 140 I had with A/C, I removed it and gained the benefit of a large, useful, drop-down glove compartment/storage bin.