The thing about a mustang Dom, is they have a great factory suspension setup. They just work. That's why so many LS swaps are out there. It's the easy way to get good sixtys.
An A-body has a crappy rear suspension, it's horse and buggy technology. It can be made to work but it's never going to be as good as a factory triangulated 4 bar setup.

It takes a great shock to overcome the lack of technology in mopar stock suspension.