Hello tcobra,
According to a Mustang tech at Maximum Motorsports, it appears that a panhard bar system can be used with a 4 link rear end. He thought that their 8.8 lower left shock mount for the bar might be the same as the 7.5 rear, don't know for sure. The frame bracket would need to be welded in. They could provide parts. The bar itself would need to be cut to length and drilled and thread tapped for the rod end. They mentioned cutting the right/correct end of the bar to allow using a right hand tap for threads, allowing for bar length adjustment. Most of all they would want photos of the rear end and frame with measuring tape showing the dimensions, so that they could determine best parts needed. I'll keep it in mind for the future.