Truck rallys and disc brakes problems

[howzzzit]

I know there are several posts about this subject, but here we go again. I have a set of 15x8 truck rallys with 4 inch backspacing. They will not mount over the original brake calipers. Truck is original and original ride height with no modifications. It's been stated this type of wheel is compatible with disc brakes as long as they aren't larger aftermarket calipers. I have a 72 C10 with disc front/drum rear. What gives? I haven't tried them on the rear drums, but if they won't fit the front they are useless to me. Bought these at a swap meet and a lesson learned. Any thoughts? (other than I'm screwed) Thanks
One thing I notice is a slight inward taper of the rim toward the hub. Normal?