A propulsion system for a tiltrotor aircraft includes an engine supported by the airframe and a fixed gearbox operably coupled to the engine. Inboard and outboard tip ribs extend above the wing and define slots. Inboard and outboard bearing cartridges are received in respective slots. The inboard and outboard bearing cartridges respectively include inboard and outboard bearing assemblies. A pylon assembly is rotatably coupled between the inboard and outboard bearing assemblies. The pylon assembly includes a spindle gearbox having an input gear, a mast operably coupled to the input gear and a proprotor assembly operable to rotate with the mast. The spindle gearbox is rotatable about a conversion axis to selectively operate the tiltrotor aircraft between helicopter and airplane modes. A common shaft, rotatable about the conversion axis, is configured to transfer torque from an output gear of the fixed gearbox to the input gear of the spindle gearbox.