I have a 72" old heavy duty rear blade on our 43hp JD. Its not hydraulic just runs of the 3 pt. I use it for snow,gravel drive, and smoothing out the dirt horse pastures. The tractor has plenty of hp for that blade. I would measure your width of your rear tires and get a blade as wide or just slightly wider than your tires. If you have vegetation on your trails these blades wont work very good, it will just ride over it. A box blade would be better for that. You can dig a little more and carry more dirt in the "box" part. I don't know if its possible but the right way to fix that spot is to take your FEL(when its dry) and cut that crappy wet topsoil out of there. Throw it to the side and build it up like you said. Try to get the water to one spot. At this low spot put a small 6" tile or culvert here. Then put some good material in where you cut the road out. Gravel or some type of good clay,no black dirt. If you don't get rid of that soft crappy dirt it will always be a problem here.