This is why I went with the 9 inch Ford straight axle. But there are options to beef up your IRS rear end, and other tranny options too (I am going with the Richmond Overdrive 6 speed, and figure the right lube should help in there, after break-in) also using the latest Centerforce 2 dual friction clutch and all related goodies.
Rear end options:
http://www.cwiinc.com/ (This is one bad A$$ rear end!)
http://www.cobracountry.com/amp/home.html
AND:
http://www.performancemarket.com/rearend.htm
Hope that helps. The 31 spline standard axles are nowhere tough enough for your app. I went with Strange Engineering 33 spline *Street Pro* axles, and SE posi modular differential.
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