I just did this swap on my truck. My advice is definitely plum bob your center pin off your upper bump pad. Mount the chevy leafs on your axle with the mounts attached to the springs. Line up your axle center pin mark with your plum bob. Once every thing is in place then tack up your mounts. Take lots of measurements. I wouldn't recommend doing it if you going to be in a hurry. You always have to go back and do things over. I definitely wouldnt run dual shackles in a road worthy rig. This write up might help also.
http://www.pirate4x4.com/forum/toyota-truck-4runner/20665-63-chevy-1-2-ton-spring-swap-faq.htmlHere is a few pics of testing them out.
Here is a
