From your photos it looks like that C channel frame is the same thickness all the way back to the rear of the trailer. The caps or the return flange take most of the load, the web is there to stabilize the caps. I would design the caps to be thickest over the axle and taper off as you go aft thus distributing the load away from the stress point directly over the axle. The dynamic load may be 10 or more higher than the static load to put that channel in bending. A box beam would be the strongest followed by an I beam and then a C channel for strength to weight. I think your plan of cutting the flange to straighten and jack up is a good one. I would just add a plate or two to the lower return flange to distribute the dynamic load better. Adding a plate to the flange to stabilize the caps would,be a good idea also. Hopes this helps.
