Ties the front suspension strut rod mounts together. Was Ford's solution to the Noise-Vibration-Harshness (NVH) problems in the early production Pinto and the StangII prototypes.
Be a lot better with it (or a substitute fabricated to fit) in there. Should also help handling to some degree, as it will help keep the lower control arms where they belong.
The good news is that every II ever built had one, so don't pay a lot for one. If I had any left, I'd send you one, but I have not been keeping them when I part cars.
It's the gold piece directly behind the engine crossmember. they can be a PITA to get access to to tighten the bolts.