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See if you can find a data plate for the chassis showing the rear axle capacity. I'm thinking they were built on a 2 1/2 ton chassis. Similar to what full size school buses are built on in the U.S.. What engine does it have? Is the distributor in the front or rear of the engine. If it's in the front, there will be some numbers stamped on the flat section under the distributor. These will give you the engine size. You probably have a heavy duty Torqueflite transmission called a Loadflite.