I consulted my 73 3500s owners handbook on the above. I was amazed to find it gives a bewildering range of loads based on gradients, ( 20,25, 33.3%, etc.) and altitude ( 1000,
6000,9000 feet above sea level,etc). I would say that provided you avoided Mt.Snowden and the Swiss Alps and you had a trailer fitted with brakes, you could safely haul 1250kg (25cwt). I have successfully carried greater loads over short distances without any problems (unbraked). As long as you remember you are towing a heavy load and drive and brake accordingly, you should not have any problems. The P6 and the SD1 were always touted as good towing vehicles.