8 lug to 6 lug

[95 S_Trucker]

If the truck is a 99, Its a C2500 LS. It doesn't have a Silverado badge anywhere on it. It would be 8500-8600 GVW. The rear axle should be a corporate 14 bolt 10.5" full floating axle.

For the front, you can swap all control arms and knuckles and brakes from a 88-98 2wd 6 lug truck.

For the rear, you can swap a different axle. You may need different u bolts if the tube is a different diameter. You will probably need a conversion universal joint.

If you swap to disc brake axle, you can remove the 15PSI residual pressure valve from your ABS unit. You will need 2PSI residual pressure valves to prevent caliper pushback. They can be mounted anywhere in the rear brake line circuit. It doesnt have to go in the same place as the 15 PSI valve. You can use your original brake master cylinder and ABS unit.

The largest GM trucks to have 6 lug in this era had 7200 GVW. Keep that in mind as you will reduce payload and braking effectiveness. Your c2500 weights around 6000 pounds empty. That doesn't leave much for payload or passengers

It may be better to look for a set of 8 lug wheels you like.