CB,
I have the 1992 W-250 with rear discs and the 1993 W-350 with rear drums. I might get kicked for saying this, but I think the drums (properly setup) are still the better brakes. Empty, the W-350 will easly lock the rear duals up, but the W-250, you have to really mash the pedal and then the fronts usually lock up first. With a gooseneck on the drums still stop better than the discs.
On the W-350 I have 1" wheel cylinders. The master cylinder has a set volume and pressure that it provides. So with the 1" wheel cylinders versus the 1-1/8" wheel cylinders, there is more force exerted on them. Now, the same thing about the master cylinder applies on the W-250 with the discs. The MC puts out the same pressure and volume. The rear disc calipers require more volume than the wheel cylinders, so the braking is not as aggressive as it could be. I have developed this conclusion after discussing this with a friend or two. If you have the money, just redo the rear drums on your truck and you won't have to mess with them again. Just providing my view/opinion, not trying to support/not support any companies. Hope this helps.
Jeremy