My MMS rear diff has machined grooves into which O'rings are seated, just as tahustvedt has done!
Justin - I can't believe you are running your ProCat with a 5.5bl!!
- I feel that mine is just on the edge of what is acceptable with a mild (Old Hacker motor) 6.5bl... which if I'm honest is quite quick enough!
My ProCat is running the slipper/layshaft assembly & rear Pro-diff from a comp-spec BossCat - also the MMS front diff pulley - so far (touching wood) I have not experienced any belt slip in the four-five meetings at which its been run, although I would say that the main/long belt is probably set tighter than I would if I was running it with a brushed motor.