I used a section of 4" pipe to put a notch in my existing frame after I boxed it with plate steel...3/8" I think.
Here is a good read on strength of such a notch.
http://67-72chevytrucks.com/vboard/s...d.php?t=457037
After understanding this information, I would go back and build a small bridge section on top of the frame rail to provide additional support. I remember Ogre did that on his truck.
Search C notch and you will see a number of discussion threads. It may get you where you want to be with your truck with a lot less work and fabrication. There is plenty to do all around on one of these builds.