If the local codes allow it, they are used often as a source for radiant heating in highly insulated homes today, however the systems put in 51 years ago were installed as high temperature units (higher than a water heater is meant to serve.) The homes 50 years ago were poorly insulated, meaning that the heat lost by an old home is often three times more than a new one. The water heater would run 12 times more fequently when acting as a boiler.
However, there are water heaters that are certified for boiler use as well as domestic water use. They are as expensive as a boiler.
You can find such systems with complete instructions if you do a net search for radiant, water, heaters, boilers, etc.