I may have missed something, but isn't the pupose of the adapters so that the original Zed halves can be used?
I've done a little research and 3.9 diffs are in the 01-06 STi's. Before this they were 3.44 and after they were 3.54.
Gotitrex has the 3.9's for $750 (93180550). That would mean a solid LSD for about $1500 with the US stubs. A bit of $$ but from my research and understanding a good LSD will massively improve the handling of a tweaked Zed. (Prob more that $1500 into the engine)