Will be interesting to see how your installation goes. I was hesitant to cut that much out of the crossmember and specifically avoided the aftermarket units because it seems that the crossmember provides torque resistance to the frame in these cars. That said, my kluged up transmission mount surely isn't going to provide much twisting resistance to impart to the crossmember either... I see you're an ME as your profile states, so I figure you've thought through this too, just interested in hearing your perspective on it.

My dad worked for Arvin industries which was an exhaust system supplier to the big three. When he worked at their tech center as the QA rep, he received a call from his counterpart at Ford who had noticed that the tailpipes on the (mid 80s) 5.0 Mustang did not extend out from the bumper equally. He went on to say that the Arvin system was out of spec and that's why. My dad took some production systems and put them on the spec jig to see if that was in fact true. He also looked at the installation on the car and came to the conclusion that the engine axis wasn't aligned with the frame axis and sat crooked in the car. The angle caused the exhaust to be crooked in the frame and therefore, one tailpipe stuck out farther (a very small amount). Ford told him he was wrong, he flew to the factory...they put a car on the lift, and..sure enough the engine isn't straight in the car. He thought this was pretty funny.

 

I saw a couple other posts where people had done the same thing and said it worked without issues. That rear plate is 3/16" and it is welded to the top and bottom of the frame member, the bottom plate is 1/4" and it is welded to the back, front and bottom and the two sides are 3/16" that are welded all of the way around as well. Not sure the ME degree played too much of a role here, I did not do any calculations, just kind of went with my gut.