[LH Lead-Foot]

I rebuilt the 1967 OEM 6 lug rear end to a 3.08 and disc type posi, new bearings & seals... I have done dozens over the years, but I also have a housing spreader to make it easy. Using bone yard spindles from 91-98 HD C1500 (Ext Cab) / 2001 Express-Savana, to get 1 1/4" rotors with 6 lug pattern for about $75. With a 51 year old truck, I choose to rebuild the front end to get disc brakes and new bushings to last another 50 years. Using new G20 lower control arms, shaft & bushings, calipers, GM tie rod ends & ball joints, ACDelco hoses and bearings from Rock Auto. Calipers came power coat red for $40 each which are GM calipers with 2 15/16" pistons. All for $300 new except for booster, bracket, master and proportioning valve that you'll have to get.
The problem with 03 wheel from a GMT800 is the major positive off-set from center to cover the hub/bearing assembly as with most new vehicles. I believe you will be hitting the control arms or frame depending on what disc brake conversion kit you buy.
My wheel purchase was pacer 16 x 8 that have a +10mm off-set from center to keep the tire at zero scrub. Yes, hub rings are required to center & support the wheel.
I did not have the $2,000 - $3000 to spend on front disc brake kits, just went with all off-the-shelf GM parts as the lower control arms and rotors are used on 3/4 ton and those labeled heavy half. I use stock height spindles that are found clear into 2001 on the Express / Savana. They use the same 1 1/4" rotors, they just come with an ABS tone ring on them.