There looks to be plenty of meat behind the rust. Sandblast it while apart. Heavy zinc paint then enamel and bolt back together with 8.8 tensile bolts.
My two cents- if the truck wont be doing hard work. Check for more cracks as above, vee out the crack halfway through from each side, and get a really good welder to fix it. They will know how to minimise distortion in the frame.
You may opt to add a fish plate (long diamond shaped patch) over the repair for extra strength.
And keep an eye on it.
