I would use the 3905 diff with the flm al cup and some sort of al case.
if you use 3905 diffs with the flm 3mm diff cup, theres only one company that makes a AL diff casing, which is golden horizon. heres the cheapest place i found them http://www.rcplanet.com/ProductDetai...H02584&click=3
I did this myself, actually was going to sell the required parts to do the swap, but did not find a suitable front case support to modify to fit the lst case. And I did not feel like making them myself...
If you want to some (bullet proof diffs) get these.. I have them and they are great!!, i been running run them on my GR3 E-Revo on 6-8s for the past 8 months and have had no problems at all with them..
id get the 3mm screws for the extra $15. let me know if you find another company that makes AL cases other than the golden horizons one that i linked. i think i remember reading someone had to made a few mods to make it fit correctly.