Find the height of the centre of mass of the weight and cart. Draw a straight line from the COM to the outside edge of the cart’s wheelbase ($250$ mm from its centreline). The angle that this line makes with the vertical is the maximum angle that you can tip the truck. An angle of tip greater than this will take the line of action of the weight and cart outside of its wheelbase, causing it to topple.
Note that this assumes the support for the weight is rigid and the weight is fixed above the centre line of the truck and will not move as the truck tips. It also assumes you are moving the truck slowly, so we can treat this as a statics problem rather than a dynamics problem. If this is a practical situation then you should allow a generous safety margin on the maximum tip angle, and also consider ways to lower the COM and make the cart/weight more stable.