THE Transmission Thread 2.0, Now with Pictures

[Scargo]

I have a transaxle rebuilding question. I an putting close-ratio fifth and sixth gears in an '04 tranny. I have it apart to the point of the driven gear cluster and the main shaft with gears.
I need to know how I can safely hold the driven shaft gear cluster so I can remove the nut that holds it all together. It requires 400 foot pounds to loosen/tighten the nut. It's not a matter of having or obtaining sockets or wrenches to apply leverage on the nut, but how best to hang onto the gear cluster while I break the nut loose. I only have a 1/2" impact.

EDIT: Nevermind. I sprung for an El Cheapo 3/4" drive impact. I made a punch tool to undo some of the four indentions of the nut's locking ring. Having an assistant hang onto the main shaft with a chain wrench around the the middle of the fifth/sixth gear set, I was able to remove the nut fairly easily. There's still a lot of metal particles scraped off the nut that you need to be aware of as you clean it and proceed with the gear swap.