I swapped in a 14 bolt into my '69 GMC (a long time ago so hopefully my memnory is still good). It was older than the one your using and it was a FF. I had to change the spacing between the springs by 2". Rather than cut the pearches off, I cut the fivits off the hangers and made 1" spacers to between them and the frame. It wasn't that bad of a job.
Going to a newer axle, especially if it already has disk brakes is a good upgrade. Although I'm more a fan of the FF. Here's a good link to 14 bolt info. Been a while since I've looked at it so not sure if there's much for SF info, but take a look. Might be helpful.
http://www.pirate4x4.com/tech/billavista/14b_bible/