Brakes are too important to do a poorly conceived conversion.
Looking at your pictures I'm wondering if you'll have enough clearance around the spindle and tie tod. I'd suggest you confirm that if you have not already done so.
A 6400 is a 16,000 GVWR truck, so you need serious brakes up front. I think I'd have considered welding up the holes on that hub and redrilling, the approach your local machine shop rejected. They probably know more about it than I do.
Have you considered machining new hubs?
