[AussieinNC]

My understanding has been that the difference between
a standard TH400 and a TH475 is the TH475 has some
straight cut gears that are noisier and stronger and
that the TH475 has support for a tail-shaft e-brake.

I have seen more straight gears blown up than I have
the helical gears so I wouldn't get too hung up on it.
If you ever looked at the aftermarket gearsets for racing
you will find that they have more pinion gears but ALL of
them on the market are helical rather than straight,
despite the fact that straight gears are easier and cheaper
to make.
My guess is the factory went to straight gears in high load
applications to reduce thrust loading in the gear train,
rather than any inherent strength advantage of a straight gear.
Guys who build performance 400's simply rollerize the output,
making the thrust loading a non-issue.