I ended up buying a complete set of drum brakes from a 1996 Chevy G30 Van. I got the backing plates, the wheel hubs, the drums, and all of the components off and into a wheelbarrow. The parts should have cost me at least $70, but the lady who checked me out only charged me for a brake disc with hub. I didn't care though, because it meant I got the bulk of the conversion parts for the low, low price of $20.99.
Besides those parts, I ordered a set of parking brake cables and new wheel cylinders from RockAuto for about $40, and spent about $5 for adapters to connect my existing 7/16 brake line fittings to the 3/8 fitting on the new brakes. All in all, I think that I got a pretty good deal, considering how much bigger the new brakes are (13 inches instead of 11), and how much easier it will be to get parts. Right now, I am just waiting on my new wheel cylinders, then I can finally install my new brakes, and get my truck to stop and have a parking brake.