It sounds like your either going to have to run some over or hand spread it....I know my atv spreader throws urea like 30 ft....so in theory you would only need to run a little bit over. Also are you top dressing for a reason and just wanting to? I feel like 150# at planting should be sufficient to get great plant growth. Also remember that if your not going to time it with a rain you will have to use treated. 46-0-0 is what i would say but really anything high in N will work. Good luck and be sure to take pics when you spread and then again a month later. Log it to see if its worth it in the future and post up your findings!