Put a chunk in it with a standard differential. Then stick a tape measure in until it hits the shaft in the differential and measure to the end of the housing. Do this on both ends and then call Moser with those measurements. They can make your axles by these measurements. You will also need to know what differential and brakes you are going to use before calling.
I would recommend subtracting about 1/16 inch from your measuerments so that the axles are slightly short. That will give you room for backlash adjustments when setting up your gears.