Michael,
Although I have no idea what cornflakes (Kellog) have to do with
grammar, here are my two pfennigs worth:
 
  (1)  If you were doing an old-fashioned Kellogg-Reed diagram of it,
  where would the intensifier "itself" go?  As an appositive for
  "building" or as a modifier of "building"?
Appositive, I'd say. Appositives (the building/itself) refer to the same
referent, often they can do so each on their own (Michael/Kirschner),
while modifiers give additional information on what the head refers to.
 
  (2)  And what is the subjective complement in the sentence -- "stories"
  or "high"?  I think it is "high" wqith "stories" modifying "high" and
  "four" modifying "stories."  Right?
The subject complement is what I call a 'special group' (for want of a
better label), where 'high' is the head and 'four stories' is the
attribute (old-fashioned) or modifier (old-fashioned too, although a bit
more recent), which is a noun group (or noun phrase, if you prefer) with
'four' as the modifier of 'stories' - just as you said. This type of
group is quite frequent: three miles long, 20 years young, etc. - the
attribute expresses some kind of measurement.
 
Burkhard Leuschner