I had good luck using Marine-Tex rather than J-B weld on a crack in a Pontiac straight 6 block. It is a two part epoxy like J-B but seems to tolerate moisture better. I have pictures that show the still seeping crack, the repair, and one a year or so later that shows no leakage. The crack was over a foot long. Marine-Tex is very expensive so if there is a boat repair or Corvette shop near you they will sell you a couple of spoonfulls which is enough for the job. Just don't get it till your READY to use it.
