A while back I was talking to Carl Wade at S&S Automotive, in Denver. He pointed to a big block Superformance. It was there to have a cracked differential case replaced. Again. That was the third one. In his (and my) opinion, the problem with the SPF is that the front mounts are rubber, but the rear is solid. One end is allowed to shift, and the other isn't. Maybe that's why yours cracked????
I have a FFR with IRS. It's the same basic set up as yours. I have a big engine that makes big power, I run it on the track a lot, and I'm not shy about hitting the loud pedal on the street. In short, I beat it pretty hard. No problems with the diff case after 6,000+ miles. During the build, I installed solid alum front bushings.
http://www.vintageperformancemotorcars.com/
I considered the Steeda brace. But the only dramatic case failures I'v seen were in high powered drag cars. I'v not heard of a Cobra having that same catastrophic failure.