first off, welcome to the FTE. it's a great place for you. I would also recommend you check out the large truck forum here. You might find big truck answers more readily there.
I was once told that 'expert' dissected is 'ex' as in formerly and 'pert' as in happy, so seeing as I still can smile and laugh, I would shy away from the title myself. And though I'm not an owner (yet) I privy myself and avid fan of the mid 50 Ford C series trucks.
have you googled the topic?
try a variety of terms such as 'fish plate' and 'frame splice'
there's a bit I've found in searching the subject in the past.
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.