Cool truck!
Since the portion of frame appears to be a simple c-channel shape at the end of the frame I'd be inclined to replace that section of the frame by having a new piece formed from steel sheet.
This eliminates the old fatigued and rust pitted steel and replaces it with new steel.
However you weld repair it you can buy a NDE dye penetrant kit at your local weld shop that will allow you to inspect the repairs to ensure the cracking is in fact repaired. This is a good idea as many times the cracks are not visible to the naked eye.
Google "nde dye check" for some videos.
More here:
http://www.aws.org/itrends/01-04/penetrants.htmlGrainger has a kit:
http://www.grainger.com/product/SPOTCHECK-Penetrant-3WU63Hope that helps.
John
