Ace,
That device will not touch these stumps.
Jim I no longer have the tractors or the device. I am sure I have the same pine trees that you do here and I had to deal with them with my tiny John Deere. Most of the trees here in FL have a rats nest root system because the top soil is so thin. If you are using a back hoe dig a trench 4-6 ft out from the trunk and wash out the roots with a pressure washer or hose. Use a chain saw at the trunk to cut off the horizontal roots. once these are gone you can cut off the tap root or just 6 inches below grade. You mentioned this land will be used for pasture not a garden so you don't have to go that deep.
The three point is designed to pull more then the capacity of the tractor. When that happens the wheels spin. Actually it is an advantage because it loosens the dirt on the top roots. Yes I have buried the wheels on the tractor many times which is why you need two tractors.
There was a time when people cleared land with horse and oxen. The difference between now and then is effort and time.