Unfourtunately the GM swap is not a good one for the Westie because the intake
sticks up above the deck lid by a good five or six inches.  So if you went the
GM way you would eliminate your bed.  The subaru way has not been done by
enough people or driven long enough for anyone to know if it is actually more
reliable than the waterboxer (the parts are pretty expensive for that motor
too I believe).
The only swap that has been done for several years and has been pretty much
proven itself is putting a in-line 4 cylinder Golf or Jetta motor into a
Vanagon.  Many people have done this swap.  It is not that hard if you have
the parts from a diesel Vanagon that are needed.  It has proven to be reliable
(of course this depends on how well the swap was done).  And if you ever need
to replace any parts or even the whole motor they are plentiful and rather
inexpensive.
So that's my 2 cents.
Ken Wilford
John 3:16