Before you create a problem with high gear clutch failure that will be very difficult to diagnois check Dizusters archived thead on this subject. There are many different stack-ups for the 904/999. We burned up high gear clutches for a year before we or anyone else could figure it out. It turned out that the last clutch plate in the stack only sat on the inner lugs of the input shaft by about.020". At higher rpm the disc would jam on the lugs and burn the clutches in the pack. When the rpm came down the clutches would engage. We asked many experts and no one had an answer. Finally we noticed very slight damage to the inner teeth of the last disc only. It was a incorrect match-up of reaction shaft support,input shaft and front drum. Many times the supplied parts for this swap come from a lock-up trans and need a different input shaft. Beware until you know how to identify the correct parts.
Doug