I Never could find these but supposedly there are some mid 80's front brake rotors that have 6 lugs.......find a napa store with a old timer at the parts counter, maybe they can find them.......the 5 lug hubs and rotors you have are only $17.00 each new at Autozone....or O'reileys......Kieth
ps I never could find them-------------some where on this forum another guy did find them.....
You do realize you will not be able to use stock 6 lug wheels with the disc brakes. The least expensive option is to have your rear axles drilled for 5 lug brake drums (and the center hub turned down) I had them done for 100.00 and I provided the studs and brake drums.
you can see the modifications here, just click on the thumbnails to see the progression of changes. Kieth
http://kieth.smugmug.com/Trucks/66-GMC/i-8bbRfcq