I cracked my exhaust manifolds, which water runs through, on my brand new boat back in 1987.I ran the motor in March on a warm day 60 degrees, not thinking it would fall below freezing again, but it did.I used Marine-Tex which is probably similar to JB weld and they're still holding 20 years later. I think it will work on your block with no trouble.
Joe

54 Triumph TR2(rhd)needs total resto.
60 Apache 10 stepside shortbed
99 Excelsior Henderson