An impact wrench will not properly torque a bolt !!! And to torque to 450 lbs. you would need a big wrench.
Ft/lbs. is a simple formula. I weigh about 190 lbs., so if I put all of my body weight 1 ft from the axis of the wrench, that is 190 ft./lbs.
Try this: Turn you hitch 1/4 turn in the receiver so you have to push down to tighten the ball nut. Take your body weight and divide by 12. With my weight at 190, that figure is 15.8. For me to torque to 450 ft./lbs., I need to put all of my weight on a point 28.5 inches from the axis of the wrench (450 divided my 15.8). I will get a metal pipe and mark it about 28"........I need to take off about 1/2" since the bar will not go all of the way to the axis of the wrench. I will then put my hand centered over the mark, put my knee on my hand, and put all my weight on that knee....a little balancing act is needed.
You will probably need a 3/4" breaker bar and appropriate socket to use this method. If calculated correctly, I betcha this method is more accurate that a torque wrench.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263