6 lug 68 chevy is a 6 x5.5" bolt pattern. im changing my 68 c10 over to 5 lug right now as well.. there are a few ways you can do it. as mentioned above, the 73-87 spindles/brakes will bolt right up. that gives you a bolt pattern of 5x5". also, 71-72 c10s came with 5x5" disc brakes up front. if you want 5x4and3/4", it gets trickier. for 5 on 4 3/4", you can run your stock drum brake spindle, 1970-81 Chevelle rotors, 1969-72 Camaro Calipers, and all the wheel bearings from the chevelle as well. this will give you 5 on 4 3/4" bolt pattern. you will need to make a spacer that fits between the spindle and caliper. Classic Performance Parts in Anaheim, CA just came out with this kit with everything you need for only $375. heres the link,
the only problem with that way of doing it, is you cant use drop spindles. that will only work with the stock 1/2 ton drum brake spindles. thats why i didnt go that way. im using disc brake spindles off a 71 c10. those brakes are 5x5, so i popped out the lug studs, milled out the back of the rotor, then drilled my new 5 on 4 3/4" bolt pattern. ive got a new axle for the rear that is 5 on 4 3/4" so thats my only suggestion for the rear. hope this helps..