3.5 tonne is the maximum gross weight, i.e. the combined weight of everything, including the vehicle, driver, fuel, load, wheels (you get the picture).
I would imagine you should be OK with the weight to which you refer. Check out your handbook to be sure or speak with the manufacturer.
Baffled tanks are a must for liquids, otherwise you'll end up rolling. Also, be mindful of everything else on board.