Howdy,
A few years ago I read a service bulletin from FORD on how to extend and/or shorten a truck frame.
I don't have it at my fingers but I did save it and when I find it I'll post it here.
I am sure that you can get it from Ford however. It's not rocket science and there appears to be a "standard" way to do it so the welded piece is every bit as strong as (or stronger than) the original frame member.
Cheers,
Rick