Kirkham actually has built the diffs for Shelby at a number of points over recent years and they are nearly identical externally. Internally, Kirkham has strengthened the axles and gone to Trutrac limited slip, which is stronger than the Auburn LS that Shelby still uses. In fact, after breaking an axle (due to a metallurgical flaw in the axle itself, not an LS failure), I ended up buying a Kirkham diff as a replacement as they are visually identical and stronger. And removing/installing them is a massive PITA, so I'd rather not break it again.
I had a 3:31 and now have a 3.54. I like both, but probably the 3.54 better. Shelby offers both ratios I believe (as does Kirkham).
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