I would carefully record the height you jack up the frame from a fixed reference point and then retract the jack.? Then check the results to see how you have progressed in making the frame straight.? If more bending is needed, I would go a bit higher up with the jack, retract the jack and measure again.? After a few of these cycles you could get to the ideal point of desired bending.? I would go slow instead of fast in this process, so you don't overdo it.? This is how I bent an axle straight years ago.? If you add some metal optimally it should help to prevent a reoccurrence.
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Dave has experience with welding on strengthening plates as I recall from his article.? Increasing the depth of the channel would provide maximum affect for dealing with any high bending moments placed on the frame above the axle.
Brian Hovander