do not bother with the phantom grip or any other type of "insert" LSD. Get something escellent like a quaiffe and if that is too much, get the next best thing in the Retrofit. Nothing else, believe me you don't want those inserts leaving a ton of metal shavings or coming appart in your tranny... not pretty.
Do not forget motormount inserts and a good suspension.. otherwise all that HP is useless.