That piece is called the apron and if you can find a donor car you can replace it by unbolting it. Some cars have the apron welded in but I believe your 67 Mustang is of the bolt on variety.
Finding a donor is the trick unless they make repros of that item? <img src="confused.gif" border="0">
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo