A quill shaft is configured for transferring torque and accepting misalignments between a fixed gearbox and a rotatable spindle gearbox in a propulsion system of a tiltrotor aircraft, the quill shaft includes a first splined portion configured for coupling to an output gear of the fixed gearbox, and a second splined portion configured for coupling to an input gear of the spindle gearbox. The spindle gearbox includes a rotor mast associated therewith, the spindle gearbox being rotatable so that the tiltrotor aircraft can selectively operate in a helicopter mode and airplane mode.