[URL="http://www.mustangsplus.com/xcart/product.php?productid=5065&cat=2320&page=1"]http://www.mustangsplus.com/xcart/product.php?productid=5065&cat=2320&page=1

is this the part that the battery tray bolts onto??

 

That's the driver side. You need the passenger side.
http://www.mustangsplus.com/xcart/product.php?productid=5066&cat=2320&page=1

Left side/right side can be confusing but is generally defined from sitting in the driver seat looking forward perspective.