Are You Sure?
How was it determined that the block is cracked? The climate in your area isn't typically severe enough to cause this kind of damage. While I'm not experienced with fresh water systems, I would think that they, by their nature, would not be prone to freezing. Further, a cracked water jacket in a fresh water system is typically not under enough pressure to be of consequence. A layer of epoxy on the exterior may provide an adequate fix.
Assuming that we are dealing with a water-jacket crack and the system is recirculating, BarsLeak is a very effective product. They make a version specifically for cracked castings, IIRC. Unless you're going bluewater, which your proposed solution implies is not the case, you may be well to the good to give it a try. Easy, cheap, and hopefully effective.
I'm tending to believe that your problem lies elsewhere, unless hot water is spraying from an obviously cracked water jacket. And then you should confirm that the problem is not a corroded freeze plug.
If your evidence is oil in the water or water in the oil, you could be dealing with a blown head gasket. In an engine this old, it would be a very real possibility. A compression test revealing similarly very low cranking pressures in two adjacent cylinders would be confirming evidence.
Just my thoughts.