RE: What do you put where in a Westy?
Lisa, I don't think there is really an answer to your question, you will
work it out yourself.  Using/living in a Westy is   evolutionary  , not
revolutionary.  Everyone tends to develop an arrangement and style that
fits their expectations and needs.  But you are going to have a new and
wonderful adventure in the process.
BTW, your mechanic was probably concerned about the chains to the far left
of the under seat storage because ('86 and later) the ECU is right there
and needs ventilation.