Check out the following link:
http://www.fordification.com/tech/discbrakes01.htmThe easiest way is just to find a disc-brake truck comparable to your existing truck. That is, don't try to put F250 parts on an F100, etc. Then just drop the entire front suspension and take everything, making sure you also get the proportioning valve. You might also want to grab the lines, just to use for reference...I highly recommend not reusing junkyard truck brake lines.
Don't waste the time and money converting the rears to disc....they're completely unnecessary. You'll have a good chunk of change invested into something that won't give you any noticeable benefits. A front-disc/rear-drum setup will easily put you through the windshield.