Freeze plug problems

Took the 69 cuda (408 stroker) to the track this weekend. I put 7 runns on it friday in test & tune. First run on sat. I blew a freeze plug out of the back of the engine, I hope thats all it did. I will be pulling the trans tonight to be sure. Has anyone had this problem if so what did you do to keep this from happning again. I'm lucky that I didn't get loose and hit the wall. Any input would be great. Thanks Tom