I have used JB Weld on a friends oil pan that had about a 2" crack or so in a car before. It held up for 2 years and about 20k miles with no issues. That was cheap cast aluminum and it held up very well, never leaked.
However, I wouldn't have done that to an engine block. I would pull the motor and either have it professionally welded and then pressure tested/rebuilt, or find a "junkyard" motor to put in its place.