That sounds like a good plan.   I'm no antenna expert but I did accumulate a few data points that I think might be beneficial to the group:

- define "good".   Good is it works if you have line of sight to the transmitting station.   e.g. 100 miles at altitude.  my cessna installation is not "good".

- I also have a Bob Archer in my tail.   From talking to several, if you have an excellent installation you'll get 40+ miles range out of that but there's a null directly in front of and behind the airplane.  Bob Archer himself admits the null issue. As long as you never fly direct-to you might be happy with that.

- Belly installation works best on a IV.  Ideally you want a 22" radius ground plane although slightly less seems to work.

- Tim Ong did some tests that showed on a carbon airplane you'll do somewhat better with the ground plane OUTSIDE the skin.

- Bob Pastusek has a belly whip mounted slightly behind the front of the gear doors with an 18" internal aluminum ground plane connected to an SL30. His reception is "good" by above definition.

- I had a similar installation connected to a gns530W.   My reception had some prominent nulls.   I installed an external ground plane.   We'll see what happens.

- connectors have some signal loss.  Diplexors have significant signal loss.   Switches have some signal loss.  I think I read somewhere on Bob Archer's web page that the loss from a diplexor is about a 50% reduction in range.

- Get Jim Weir's RST-8020 antenna installation book.  Even if you don't build your own antenna, it's really good basic easy to understand theoretical background that will improve whatever you do.  It's available here as part of his kit for $29.  http://www.rst-engr.com/rst/catalog/airplane_antenna.html   He may be willing to sell just the book for $10 and he may be willing to email you the .pdf .   Mail order from Jim may take a few weeks.

- somebody asked about using a nav antenna for a com antenna.   The antenna length is optimized for the wavelength used.   e.g. a com antenna should have a vswr of around 1.0 at the middle of the com frequency range.  com signals are vertically polarized.  nav signals are horizontally polarized.   A com monopole needs a good ground plane.  You could put one sticking up or down out of a wingtip if you provided the ground plane in the wingtip surface.

All that said, if I had it to do over again, this is what my IVP com installation would look like:

- Com 1 antenna as far forward on the belly as I could get 18" radius.  Wire directly to the radio with only the one connector at the radio.

- use a commercial whip with the lowest vswr I can find.

- lay in an external copper tape 8-pointed ground plane (like an asterisk)  and connect it all with conductive paint.    Conductive paint alone works great but there's some belief that it develops fissures over time so the copper tape makes a more reliable system.

- Com 2 also on the belly 36" or more back.    take what I can get for ground plane.   The rationale is it's better than a portable whip and probably good enough for ground ops.  (If I had a fiberglas airplane I would put it the other way up in the baggage compartment)

- Com 2 wiring includes 2 connectors: 1 at radio and one accessible that I can disconnect and plug in my portable.   This is bad for com 2 but more reliable than a diplexor and more optimal for the portable.

Colyn