When you say "...like in the Cobras" that's the Mustang Cobra, right? I think the '99's were only 28 spline and had problems. '00's and later were 31, and then you see a lot about '03/04 being "stronger" but I'm pretty sure that's just carbon clutches for better load sharing. Since we're talking the '57 tucking in a Ford IRS should be the right answer anyway, just for the fun of talking about it.