I found a repair manual, it is for the ZF6HP26 but I thought it was interesting from a process standpoint. Obviously the specs won't jive but it gives a good idea on the tear down, inspect, measure and build. Also, an idea of the special tools required, which I'm sure many can be substituted with other standard tools.
I'll try to find the ZF6HP21 manual.
Good luck with your build. I hope your luck turns around.