not to disagree, but I have seen MANY forklifts that use the same frame for 5 and 6 k trucks, where the only real differences had to do with the counterweight, and have seen factory approved added plates to the counterweight, in order to change the rated capacity.
BUT there are far more parts of the truck that get "beefed up" than just the counterweight when you move up in weight capacity of a truck, and the brake shoe arc's diameter is one that comes to mind as pretty important.
you may find you do much better with a wider drive axle footprint (extra wide drive tires, or dual wheel if pneumatic frame) as a method of safely increasing the stability.