Well, I'd have to ask what is wrong with what you have now. Worn out, too hard to steer, too rough riding or you just think you need and upgrade?
I've done a couple of subframe swaps and put a lot of miles on them but you run into the issues of getting the front end sheet metal, core support and bumper hooked up and looking right. Plus the inner fenders need a lot of trimming. You do get a low truck that drives great though.
Personally I'd look into a Jag suspension swap for that truck if I decided that I wanted to put independent suspension on it. You don't have to cut off the frame and it comes as a complete unit that should probably cost about the same as a Camaro/Trans Am unit in your area. There are a couple of the swaps on here that guys have done or are in the middle of doing and they are pretty interesting.
This is a link to a thread that OldSub started before we lost him to the big C.
http://67-72chevytrucks.com/vboard/s...d.php?t=414496 In it he shows some photos of an AD truck with a Jag Suspension under it that will give you and idea of how it would work under the 56. On the 56 with the frame being wider the rails would sit out just about over factory Jag mounting bushings that would probably have to be cut off to get things lined up a bit better.
Here is one over on the HAMB that was done on a F100 where he trimmed the excess off the Jag crossmember.
http://www.jalopyjournal.com/forum/s...xj6+suspension Again a guy has to visualize a bit to figure out how he would do it on his own rig.