The same torque that the disc will see, which is what you are figuring the number and strength of the bolts that hold them on for.
Is the same torque that the caliper brackets will see.
So, how many bolts of what size and at what radius from center hold the caliper brackets on?
Wouldn't be good if the disc stuck to the hub just fine only to find out the caliper brackets won't stay on the spindles.
Rethinking the whole project, the P30 disc brakes, same as a 3500HD, have 13" rotors, and already come engineered and assembled to work safely and reliably. At this point I think the front axle swap is still a better solution.
Grigg