There's a few ways you can do it. One would be to find a later model rearend with disk brakes already on it. In that case, and I'm taking it you want to stay 8 lug since you do not specify, a Dana 60 from a late model E250 van would be just about right for width. They're also very common, usually with a 3.55 ratio.
Another option, though it does not address the gear ratio issue, would be to adapt late model disk brakes to the existing axle. That may require a bit of fabrication to make it work.
Something else you might want to take a look at is
this disk brake conversion for the front axle, which is made to use late model GM 3/4 ton brake parts.
