Why would you want an Allison? The only one I would consider would be an Ultrashift. I don't know if you can do the swap though. Is there something wrong with your 3 peddle? I visited with the service manager at my nearby Kenworth dealer before buying my truck. He said the 3 peddle Autoshift is one of the most reliable trans you can get.
'03 Freightliner FL112, 295" wheel base, with '03 United Specialties 26' living quarters, single screw, Cat C12 430 h/p 1650 torque, Eaton 10speed , 3.42 rear axle ratio