Manifold Heater for Cold Climates
I would just plumb it in and run it. Coolant is much more stable as a heat source than exhaust heat ever was, and runs cooler under many "hot weather" running conditions than exhaust heat would have. Ford, years ago, used water heated carb bases on their V8s with no temp regulation.
If your heater uses a water valve of some sort, you will have to use "Y" or "T" fittings instead of inline or series.