I must admit I have never heard of this crank flexing or crank breaking being
a problem assosiated with the waterboxer motor before.  I have heard of it in
Beetle engines (type 1) but they have magnesium cases.  The waterboxer case is
aluminum and was designed after WWII (unlike the type I engine) so I really
doubt that crank flexing is a problem.  The only times I have ever heard of a
waterboxer crank breaking (about five instances that I can think of) each time
it was associated with a poor rebuild in which the case halves were mismatched
(either from VW or from GEX both very poor rebuilders by the way don't waste
your time or money on either of these).
That's my 2 cents on the suject.
PS I know I am going to get some flames about how that VW factory rebuilds are
good.  Well I hate to tell you this but ,as far as waterboxers go, they are
not.  They mismatch case halves and several people I have talked to that have
had thier WB motor rebuilt by them have had broken cranks or some other type
of engine failures within the first 50k miles after rebuild.
Ken Wilford
Van-Again
John 3:16