The ZF 5 speed in my truck blew up because of the DMF.
I researched about the 7.3 flywheel problems on a number of forums and decided on a solid one from Valair with clutch and kit (Southbend also has a good reputation). The forums online also mentioned the increased transmission noise that Rot Box is talking about, and a lot of people on the forums put in an extra quart of ATF (Dexron Mercon? I think?)
In talking with the rebuild mechanic at National he said both of these things would be fine. The solid flywheel should not 'shorten' the life of the transmission and an extra quart is not going to hurt anything, "just fill it with the top bolt on the PTO cover and it will be fine". I do not think it is overly loud, especially over the diesel. Truthfully I can not pick out the noise from the transmission over the engine.
If memory serves me I think this was almost exactly one quart more, from 4 to 5, maybe, its been a while.
If you need interior grey seats I have a 40/20/40 and bench in grey in my garage that I would love to sell, cheap.
Just for FYI, there are two ZF transmissions for the Ford OBS Diesels. The early S-42 and the latter S-47. I 'think' they all interchange but I am not certain. Might make sure of this before you work on the 7.3 only to find it will not bolt to the 6.9 S-42 transmission. Again, I don't know, just trying to avoid future problems.
Cheers, Chris