go with the 23 spline . reason being for later on down the road if you decide to go with bigger tires or an engine swap. another reason being resale value peolple like seein the 23 spline in the rear case . It sounds like you already have an adapter and a 23 spline coupler so if its unused call the the company you bought it from and see about swapping couplers. I know marlin will do it, not to sure about other vendors.
