Concrete needs 28 days to fully cure. Even though "dry", keep it out of the weather in your barn for the next several weeks. Wrap it in plastic to slow the cure, and wrap it with an old blanket to keep the heat generated during curing contained. Sending it out into cold weather too soon can cause the contained water to freeze, expand, and crack your casting. Nice project ! The counterweight I made for my LX2610 was contained in an old electrical panel, and calculated out to about 650 lbs. That, combined with my loaded tires, has never let the rear end come up off the ground. Best wishes !