Alan -
Are you going with a new truck or used? I would think if you are leaving a Allison 6 speed automatic as a option it would be new. (Not going to find to many of those on the used market)
I am in the infancy stage of considering a new Show Hauler conversion (I will detail that in another thread) but if you want to keep your options to sell your conversion open to more potential buyers you will want to put a Meritor or Allison semi automatic trannies in. Many of the current buyers are racers who pull large trailers, the semi auto's will work much better in these applications than a full automatic.
Bill
2003 28' Show Hauler Motorhome on a 1995 FL 120
www.showhauler.com