I am running Posies Super Slide springs on the rear of my 54 1/2 ton and believe it to be one of the best return on the dollar investments I have made to it. The springs are the same width as stock and bolt right in. They have two versions of the rear springs with one retaining the stock axle location and the other moving the axle rearward to center it in the fender. If you are running a stock torque tube drive train you will need the first one.
The Posies part number for the rear springs is 440 with the 440A being the modified one I am using.
They also offer front springs which are dual leaf.
Shackles and bolt kits are also available.
I highly recommend Posies from personal experience.
http://www.posiesrodsandcustoms.com/posies-catalog/