I'm not Jim but we do have a custom Co-motion Mocha Co-Pilot so I'll chip in. I'm 6'2" and Marian is 5'3" so we went the custom route as standard wouldn't fit. We had ours made with a 30.5" boom tube so there is plenty of room for the stoker. This does mean that we can't use a Gates belt but we did this back in 2005 so belt drive wasn't a consideration.
If I was going to buy a custom tandem today I'd also be looking at T47 BB's, disc mount on chain stay and internal cable run for rear brake.
Co-motion were very easy to deal with and we bought ours through Tandems East