The rust pits don't look that bad to me, but also hard to tell by a picture.
The crack is on the very end of the frame right, behind the last spring hanger? if so just clean it up and weld it, only stress will be from the bed and you can probably improve on how that was mounted.
I like that bed, I'm sure common in your part of the country but not something we see here much at all.
Grigg
