I agree with the second post, you will need to drive the think to see how it acts. I would bet the fronts will lock before the rears if a standard master cylinder was plumbed like all other trucks. It takes a lot of force to stop those big disks, and with equal force going to the fronts, I can see the drums locking up.
You should consult one of the aftermarket brake suppliers for their input, but I bet I know what they will say, add disk to the front! The adjustable proportioning valves simply limit the pressure coming out of them, so reducing the front pressure maybe the only way to keep things under control. Plumbing the master cylinder backwards should let the rears activate first, and if the truck is heavy enough, it could be all that is needed.
