Does the crack go across a valve seat? Any good machine shop should be able to pressure-test the head and tell you if compression is getting into the water jacket. There is a repair procedure known as "stitching" which involves installing a series of overlapping threaded cast iron plugs along the crack that is sometimes more effective than welding depending on the location and the severity of the crack. The process isn't cheap, but it's pretty effective. If the crack involves a valve seat, the seat will have to be machined away, and a new seat installed after the crack repair.
Jerry
