I've built plenty of these junkyard spring packs using 4x4 Toyota rears as the main springs (the ones with the eyelets) and have 3 pieces of advise. 1. Have plenty of donor springs laying around so that you can add leaves as they settle. 2. never have 2 springs with the same length touching one another, they will create a stress riser on the next spring (this will be the next longer leaf) and cause it to bend right where the 2 adjacent springs stop. 3. Always use clamps that wrap around the majority of the pack, the lowest 2 leaves don't always need to be wrapped, but it won't hurt. If you want leaf spring set up tips in general I just wrote a quick how to here...
http://board.marlincrawler.com/index.php?topic=87513.msg994939#msg994939
