904 tranny questions

hey aarcuda does installing the 5 thin disc on the 4 disc clutch pac 904 is the same as the 5disc 999 , or is the 999 still stronger ?

ive been reading alot on the subject but i only seem to get more confused lol. i can get those 999 for cheap really cheap like 30bucks but i dont want a lock up converter and for what i undestand is a little difficult to make a 999/904 hybrid because of parts not matching but i like the 999 also for the lowgears so what is your opinion ?