As Steve pointed out, my preferred approach to disc brakes on an old truck is to use an entirely new I-beam type front axle with factory disc brakes, then I cut it apart and weld it back together at the correct track width.
This method, while it works great, and is simple in plan, actually takes a fair amount of work and welding skill. If you are not comfortable with this method then it is not for you, as a poor quality job could have really bad results.
I have done two so far, both work great, no problems or signs of problems with over 10,000 miles on each, and both are real work trucks. I used axles that are physically larger than what the trucks had to start with, so extra factor of safety there. I also enlisted the help of a friend who is a really good welder, I have learned a lot and will be comfortable welding the next one myself.
The link Steve gave above is the first one I did, for an AD 1 ton through 2 ton.
This is the second one, done for a slightly larger 61 ford truck.
Skim through the pictures here
http://rides.webshots.com/album/558384968mIheUEI made a point of taking pictures of the axle narrowing process and put some notes below most of them.
Grigg
