Here are some options to adding weight to your rear tires.
The plate bolts to your rim and puts the weight to the ground, not on your axle bearings. Holds your weights off of the rim so that you do not scratch them. Plate comes with a 1" or 2" shaft so weight lifting weights work great and are readily available. Must specify shaft size and shaft length on checkout.
Bolt on a set of spoof axle extensions to give your tractor the "big boy farmall" look. Can also be used to hang weights on. Drilled on the end to accept a 1/2" bolt. Unpainted.