I pulled all the way up to 8,500# class with my bone stock D17 and never ran out of power in 1st high. The front will come up even with lots of power so you need weight out in front. On a sticky track, once you find your sweet spot for weighting when you jump from say 6,500# class to 7,500# put about 1/3 third of the added weight on the front end bracket. (Make a bracket) Add to the front about 300# to 350# for every thousand pounds added to tractor. 

You should also make to wheelie bars. Allis or Deere weights hang on the platform steps on each side real well. Make sure you have the bracket that stiffens the step that goes from the step to the fender bracket to handle the added weight. You can make an easy bracket under the axle tube. There is a threaded hole on each side of the axle bearing dust cap. Just make a heavy L-shaped bracket with one bolt. Good place to hang weight. Not easy to hang it though.

Make sure you adjust you tire pressure to squat some. Take a look at your tire tracks after you pull each time to see if you are too low pressured. The tire will buckle in the center of it's track and that will be visible on the dirt. It takes some experience to get things just right. Take notes at each track so next year you can adjust. Every track is different and weather changes things too from year to year.

I stay away from slippery gravel or crushed rock tracks with 28" tires. It just tears them up too much. Another thing, on a stock D17 you have enough power to use your throttle to hop yourself down the track when your front end gets too light. When the front end comes up drop your throttle about half, too low can stall out, as the front end is about half way on the way down crack the throttle open and as the front end climbs up lower the throttle again and so on. You'll get the hang of it after a couple attempts. I've gained at least 25 feet doing just that otherwise when your front gets too high you lose traction. For stock tractors the D17 is one of the few with the torque and quick governor to make that work. I guarantee a two banger Deere or Farsmall 450 won't be able to do that with stock parts. Maybe a 450 with an after market governor, as many do have, can do it but I've never witnessed it.