Hairline cracks, especially in an area such as you're describing where there is little frame stress, are repairable.
We have done a MIG repair job to a hairline crack in the same area on a '54 1/32 ton pick-up. We used a powerful MIG welder and adjusted for full penetration. We supported the frame as close as we could to the crack and then cleaned the crack and then welded. We let it cool on its own. So far so good.
It seems likely that this might a area of considerable stress (or at least enough stress to cause cracks in the same area on a few trucks).
