Generally the reason that the stock 15" wheels will not work is that the inside of the wheel does not have enough clearance between it and the outside of the caliper on most 6 lug kits.
We recently installed a 6 lug kit on a 55 Cameo that enabled us to use the stock wheels. It has 11" rotors and does not use the stock hubs. The kit was purchased from ABS and everything fit correctly.
Fred
