Want to change input shaft from 23 spline to 18 spline

That's right, the gears are a set . Of all the gears Mopar made there is exactly one gear that can be interchanged and that is the low gear; you can swap it between the 2.66 low box, and the 2.47 box IIRC and that's that.
If you want an 18 spline input, you will need the entire gearset, or make yourself one.
But before you go that route, be advised that it is far,far cheaper to just change the disc.
Also, the early As had the 3.09 first gear in them.
AFAIK the 18 spliners were 2.44, and a coarser pitch on them to handle BB torque..That's a heck-of-a-hit for the old slanty. There was a second gearset for the 18 spliners but I cannot recall the low-gear ratio. Hah, I just remembered;2.65