prefacelift doblos are only 625kg payload, 350-400 would be all it able to carry a 500 would put it overweight,
the later ones are 730kg, i have a 350 upright in mine could fit a 500l but would be borderline overweight when full,
there was a higher payload option from fiat and you can get doblos with 850kg payloads however they are rare. check the GVW weight to find out how much yours can carry