I've seen it done where they moved the leaf spring mounts forward then cut the back off. I would lean toward this rather than cutting and re-welding a factory frame.
You could also see how these guys do it:
http://www.code504.com/s-10-frame-wh...0-s-15-frames/
Check out this video series where they shortened a 67-72 frame. I though they had a good method.
https://www.youtube.com/watch?v=O9zy08qsahk