Aftermarket uppers cause binding....so do cheap UPR bushings.
When it comes to suspension on a stang is pretty much Maximum Motorsports or Nothing. MM does extensive reasearch on the mustangs suspention geomentry and has come to the conclusion that stock upper are the best for a 4 link setup.
I just installed MM LCA and am amazed with the quality (as i am with all the MM stuff i buy)