We just went through a similar thing with the clutches in Dads 904 transmission. Standard the have 4 friction discs. "Performance" upgrade is to use 5. We managed to shoehorn 6 in. But to get that magic clearance number right, the top outermost edge of the top plate (where the snap ring resides) had to have a recess machined into it we think it was an internal hydraulic leak that caused them to slip in the first place. But the maximum number of plates is cheap insurance