I used JB Weld once. Just about lost a brand new engine because of it too. At the time I put the engine together I didnt have a welder and somebody said to just use JB weld to attach the oil pump pickup to the oil pump. About two months later I noticed that my oil pressure was almost non-existent. I pulled the oil pan and found the pickup just about ready to fall out of the pump. The JB Weld was a bad idea.
In your case I think it's just time for a new block. Water will probably still manage to find its way past the JB Weld.
Besides...if you're gonna be grinding in the lifter valley you're gonna have to pull the engine apart anyway. Unless you want metal debris in your engine.