Maybe its a freeze plug on the back of the block. This would be extremely hard to detect. And you did have a rebuild. Maybe a bad seal on the freeze plug. If your loosing coolant from your head gaskets and retorquing them fix's the problem, i would highly consider redoing the head gaskets. If it is leaking coolant from there then the damage is already done. You wont get a very good amount of mileage out of that.

!!!!!!!!!!!!!WHAT ABOUT BOB!!!!!!!!!!!!!!!