My guess or SWAG is that it is probably an AT540 4 speed Allison as those were popular about that time frame. They were not used with 292 or even smaller V8 engines and as I recall the 366 was pretty much the standard V8 in a 6500 series so I would conclude that they require that kind of torque to work properly. I'd sure try to get the complete drivetrain if possible.
I worked on some school busses around that time that had this setup and also a two speed rear. Pretty unusual and were built for the mountains out west.
I know you asked about someone having done this swap but this is all the light I can shed on it.
