My Vieluxe spit is 3/8” square throughout its length and tapers to 5/16” square at the tip that engages into the motor driveMy spit is 3/8” in diameter and drops down to 1/4” at the point where it goes in to the motor. It is approximately 46” long and is clearly original.
It sounds like you have the motor part solved. Mine will arrive from an eBay seller soon.