I came across this thread the other day, and had never heard mention of it, Apparently GMT900s have a hydraulic rear cab mount, and will fit 400s and 800s, and provide somewhat better ride quality, all in all I think it came out around $100 bucks total in parts, Going to check it out myself and get a parts total, but you guys take a look and see what you think, considering trying this on both my crews personally.
http://www.gmtruckcentral.com/articles/bodymounts.html
http://www.gmtruckcentral.com/articles/bodymounts.html