I would drain everything and remove all the water jacket plugs in the block and heads. Slowly add water to the radiator. As water starts coming out of the lower holes (and not running out the oil pan) you can replace the water jacket plugs. The the theory is the more you add, the higher up the crack/leak.
If it's a low end crack, consider block fill up to the bottom of the water pump holes.
Your quote "I work out of town a lot and we may have gotten a freeze before I put antifreeze in it. Last time I had it at the track everything was fine. Something happened since then. As bad as the leak is I think there is a good chance of the block being busted."
Any chance a son, nephew, or neighbor took it for a ride while you were out of town? Your description of water running out the bottom as fast as you pour it in sounds fatal.