Matt,
i dont wish to change from a std 3.64 final drive, i take it this isnt such a problem to source? was your problem that you wanted a shorter ratio for acceleration? e.g. a 4.4 or such
got it in one m8!!
i needed a 5.1:1
then a 4.44: , a 4.27:! and a 3.77:1 - all of which i have now !!
all i can recommend m8 is try and find the lowest mileage cwp you can , have it x-rayed , shot peened (stress relieved ) and polished !!
and you should have no problems !!
try and stay away from cossie cwps as most of them WILL have been ragged !! look into the other models