If all of your current front end parts are in good shape, another option is to buy the Early Classics 3/4 ton to 1/2 ton dropped spindles. This way you keep ALL of your current parts but can go to a 5 or 6 lug disc brake setup, just needing the calipers, rotors, bearing and spindles. You keep all your tie rods, ball joints and a-arms stock.
Oh, you can get disc rotors in 5 or 6 lug but the 6 lug are from aftermarket companies, not a stock GM part like the 5 lug.
Like I said before, many, many ways to skin this cat.
Start by reading everything you can on this board regarding disc/suspension swaps/drops.