I've never heard of a Chevy coming with a 300 from the factory.
you may be right dave, honestly im not an expert on tcases/transmissions at all, i bought several d300s along with several sm465s, i assumed they were mates when i bought them, but im probably wrong. regardless, i have one d300 tcase left whatever the heck it goes to. its a course spline, i do know thats an older model.