Jeff, I don't know why you are trying to repair the tub when you really should replace it. The crack will probably spread as you stand on it. But since you asked, you must have a reason. One thing you could try is epoxy glue. Put masking tape on the bottom over the crack, if possible. Get 24 hour epoxy and a filled epoxy like JB Weld would be best but something like Devcon would be almost as good. Be sure that it is dry inside the crack. Mix the epoxy per instructions and let it dribble into the widest parts, then squeegee it into the narrow parts using a plastic squeegee or a playing card. Let it set for 24 hours!! No guarantees on this!! And I would replace it as soon as possible. You might want to install a bedding coat of cement under the new tub to spread out the weight.