The advantage to using the truck frame is that it will be less costly to obtain, however, you have to work around all the bends of the frame. The steel channel is straight, but more expensive. Now cut two 9 inch long pieces of the 2 x 4 inch steel tube, and then cut a 1 inch hole in the center of each one. Weld these two steel tube sections to the bottom of the frame rails in about the middle of the frame. Be sure that they line up with each other.
Some grease here is helpful.
It may take a little extra time to get everything lined up, but if you made all your measurements properly then it should fit together nicely. The frame rails should now pivot on the axle assembly. The next step, is to bolt a 16 foot long 2 x 12 inch main support beam to the outside of each frame rail. It is important for there to be more weight in front of the axles, to push down on the tongue and provide traction to the tractor. The weight distribution should be about 55 percent in front of the pivot point and 45 percent behind.