My Commando uses a 56 Dommie box and as Les says, the top mount is wider than Commando. Basically the top and bottom mountings are the same width and the Commando top bolt spacer is not used as the box slips right between the plates perfectly. (does anyone know why Norton did this?)
You will also find the outer cover may have the clutch entry further back (anticlock) than Commando meaning a more difficult route for the cable behind the ISO tube. I replaced my covers with used Commando parts.
All the gears are interchangeable in any AMC box, only the mainshaft is Commando specific. I just replaced my 1st and 4th gearsets with current parts. Gearing between 1960-1973 is same as Commando when the 2nd set got a higher ratio.