Gains are whatever you want to build into the pack, main gain is 2" forward axle location for better firewall clearance: http://www.4crawler.com/4x4/4R_suspension-III.shtml#CustomSprings
You will need more than one spring pack.

Move mounts, yes or no, depends on your needs: http://www.4crawler.com/4x4/CheapTricks/CrossoverSteering/index.shtml#SpringHangerDrop
New spring, lowered spring hangers are a big +. And since you are doing a SAS, you are doing that anyway.

I imagine a few of the off-the-shelp places make 5" stock length rear sprigns, or call up Alcan (http://www.acanspring.com/) and they can make you a set.

r.c.brown@ieee.org
TruckEditor@tlca.org
Project: 4Crawler
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