Hi Jeff,  I do not know your musical and arrangerial background but
as a very general introduction to arranging, maybe my (quite old)
"course for arranging" at http://www.haraldmmueller.de/arrco_en.html 
might help you a little bit.

Arranging for larger organs like the Wurlitzer 125 adds, on the
whole, "only" two more complexities: (a) you must "hear" some suitable
register selection ("sound"), and (b) you must "hear" drums into your
arrangement.  Then you are done.  So much for the very high-level
picture... and no, I also do not want to start another project (I have
some arranging for choir, some for band, and some for a Ruth 33 organ
waiting to be done -- next to my actual job and family...).

Regards,
Harald Mueller
Grafing near Munich, Germany