Just a thought... remove the rubber stopper pad from the clutch pedal box so the clutch pedal goes up 1/4" more and see if that makes a difference.
I really wonder if your master/slave isn't "pumping up" and holding the throwout in slightly even when the pedal is fully up.
One way to check this would be after having it slip, go under the truck and push against the slave cylinder rod/clutch fork by hand. It should take some force, but slowly retract as you push on it as the fluid gets moved back into the master. When you release it, it should slowly go back to where it was originally.
Anyone know what clutch Justin was using on the Bronco? I thought that was a LuK clutch, and that was holding 400+ HP and 50 psi of boost.