I would build a steel reinforced footing (A pretty large footing not more than 8' wide) on each bank that had three slots (one in the middle and one on each side about even with the tires on your tractor) to receive three 6X17.25X0.36 I-Beams. I would then place the three I-beams into the slots and anchor them to the footings (either set an anchor plate in the footing when you pour it and weld to the anchor plate or drill into the concrete and anchor with two 1/2" anchors on each I beam). For the top of the bridge I would use Select Railroad ties anchored to the flange of the I-beams with two 3/8" carriage bolts on each side. I would then place treated 2X6 runners on top of the railroad ties and even with the two outside I-beams. These will provide some extra stability and will also mark the area which the tires of the tractor should run. This is not an engineered solution but it is more than enough to hold your tractor without having to worry about it buckling or falling if it is well constructed. Not knowing your level of skill I might suggest that you hire a contractor to perform this work as this is not a simple project.