THe "U joint" is a threaded clevis and is used on many aftermarket boosters. How this rod attaches to the brake pedal is a critical element of the brake system design. Typically when upgrading to an under floor mounted power brake system on AD trucks the entire assembly is replaced with one that includes the master cylinder, booster, pedal assembly and braket.
It sounds like you may be trying to retro fit your booster to the stock pedal set up. This may be a mistake. Look into replacing the entire unit.
The geometry of the pedal to the pushrod determines how effecient your system will be and you do not want to kluge something together in this area.
This subject was addressed in a technical series in Classic Truck mag. Check their website to identify which months cover this subject.