I haven't finished mounting mine just yet. BUT..  with the middle side window out.. 
I can "FIT" the jalousie window in the hole fairly securely. 
(it's been sitting there unattended for most of the summer as I drive the project in and out of the garage)
My idea is to cut NEITHER the jalousie or the van body,.  but to make 2 filler strips (aluminum) for the top&bottom gaps.
and use some TORX security screws to fasten to an interior trim piece i've yet to imagine/create. 
the interio panel can either be wood or formed fiberglass i've yet to decide. 
I plan on using automotive/commercial Urethane sealant for the frame to body seal. very strong hold but nonpermament.

I'll document my method when I get back into it, when all this dang upstate NY snow recededs. 

Dan in Syracuse
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Date:    Sat, 10 Feb 2007 21:16:46 -0500
From:    Pascal Giasson <pascal.giasson@GNB.CA>
Subject: Re: 

I have followed the instructions described in this link
http://www.everybus.com/dworkin/vgon_jalousie/ 
to replace my driver side slider with a jalousie out of a 73 westy but
something does not jive.  The two vertical pieces that are 3/4" wide do not
seam to accomodate the 1/4" filler piece that is supposed to be sandwiched
between them. The  1/4" filler piece protrudes out most of the way.  It
looks like the 3/4" pieces should be 1" wide to be able to accomodate the
filler piece. HAs anybody who has done this run into the same problem? I
don't have my slider out and dont plan on taking it out for a while, but
does anyone know the measurments of the slider on the driver side?  Is it
in fact 42 3/8" X 16 3/4" as these instruction say?
BTW the router and tablesaw worked great at cutting the aluminium.
Thanks
Pascal