Wheels intended for use with drum brakes often have the wrong internal shape to work with a disc brake setup as you have just found. I have the 6 lug CPP disc brake setup on my '50 truck, works fine with the 15" K5/Blazer wheels that the truck has on it now.

Stuart

1950 Chevrolet 3100 (occasionally working) truck:https://c2.staticflickr.com/6/5490/30684982201_6dc479d18d_c.jpg
1928 Chevrolet AB National Phaeton: https://c4.staticflickr.com/6/5573/30137792083_42886d30a4_c.jpg
1932 Austin Special Sports: https://c8.staticflickr.com/6/5793/30116567183_1520e67c4d_z.jpg