Trucks don't drive like cars.
The trucks manufactured meet the spec.
600 KG on the front axle will put it over the manufactures weight specifications. (full of people, full of water, full of stowage)
You can not put the tank any further forward as it will not meet spec for weight distribution.
The answer may be use a more suitable cab chassis than the Isuzu type.
Spec trucks more suitable to doing the job.
Be more realistic about what is to be carried on one truck, where it is to go and the task it is to perform.
A fundamental problem exists between the weight carring requirement, weight carry capacity, weight distribution requirements and axle loading. The government also dictates what may be carride per axle. It is a complex compromise to achieve the balance.
Andrew