As far as fixing, blazin is right. You will have to remove the infected area and replace with good solid steel. There is a cheap way and just weld something in there and weld up the cracks, but this will not last. (some people don't care they are just looking to last as long as they have truck)
On the other hand, if you are the kind of person who wants to do it right and make it last, then winky had a valid point also. You should inspect carefully the rest of the frame for cracks, especially around stress area's where the rust has weakened it.
You did not say what year this truck was, if you find more damage, will it be worth it to you to fix or find another frame ?
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