Paul,
I am just not up to speed (pun)on the latest automatic transmissions. I just assummed, (never do that) that if 12 speeds autos were availiable, surely 10 speed autos were also availiable. Then I assummed again that you selected the 12 over the 10 for some specific reason. Those darn assumptions rarely result in correct answers do they...
Standard tractor transmissions generally include 9, 10, 13, 15 and 18 speeds. I guess Meritor decided a 12 speed was needed. Volvo makes their own 14 speed available. I don't know why.
If I were planning your type of conversion, I think I would begin with a day cab too. I don't know if Warpath wishes he had started with a day cab. He may have gained some space and aerodynamic advantages.
onezman
http://onezman.tripod.com/whitehotrvhauler