I've looked online and asked around locally and so far haven't found anything for caliper upgrades for the K2500. I have the 8 lug which, from my searching, seems to have the same front brakes as the SRW K3500. The DRW does have different front brakes, but they appear to be the same size piston with just a thicker rotor.
I recently found the 3.5" drum brakes off a Burb. I'm thinking they will be a bolt on item but I havent tried yet. A local guy says he has a place where he can send the shoes for a special kind of lining that will make a big difference.
I'll have to go out and test the prop valve to see if it's working right.
A few years ago I put stainless flex lines on my Toyota FJ40 and did four wheel discs in place of the old drum brakes at the same time. I can't completely seperate one from the other though when I say the difference was night and day.
I already have a set of premium semi-metallic pads from CarQuest, not yet installed. So I'll check out the wagners.
Don