if you are not totally oposed to the westy layout, stick to it.
i am planning on a sililar project but think i will make the following
changes for easier install and better usability IMHO

a) use karl mullendoores fridge instead the westy fridge:  see
http://www.members.shaw.ca/McTavishPAC/fridge/fridge.html

b) now that you use gas only for the stove i would not install the big gas
tank under the car but go with the $2 green coleman bottles and connect
them. you get spares everywhere, dont have the hassle of the installation
and save a lot of money.

c) spare the hassle of all the outside connections (city water/110v/water2)
unless you think you really need it. i have spent months camping in my westy
and neverr needed it despite for filling the water tank, which could be done
with a hose to the interior directly just as well.

d) if you have money left buy the undercar frshwater and dirtwater tanks
from www.reimo.de  yes they are 100$ a piece but - you free the huge space
of the water tank inside giving you an extra cabinet, and your dirtwater
doesnt just run on the ground.

e) if you want to optimize the westfalia layout a little check out the
conversions of weinsberg and carthago (go to vwpix.org   - archive-  camper
- weinsberg or carthago) these are relatively close to westy but IMHO a
little better. it might be relatively easy to convert a westy setup to their
layout. note: this idea just occured to me so i havent researched it yet.

but most important of all:

have fun!

florian