I suspect you've probably already come across this but I'll throw it out in case you haven't.

http://www.ridetech.com/info/air4link-tech/

For bushings I usually prefer the rubber ones.....but with the power and tires you are going to be running I'd probably seriously consider urethane. Just make sure you get ones that can greased.

On spring rates it's probably going to be best to talk to the manufacture or you could go air bags.

One of the things to consider for the springs will be fuel tank location and size. A 20 gallon fuel tank hung under the bed behind the rear axle will probably require a bit stiffer spring than a 15 gallon tank mounted in the front of the bed.