manual box stock to fast ratio

I want to replace my stock manual steering box for a fast ratio 16:1 box.

Do I have to replace the pit arm when going for a fast ratio box?

Rockauto sells them for 285 dollars.

I hope someone corrects me if I'm wrong but the pitman arm is the same but you can alter the ratio further with a different idler/pitman arm designed to do so.

Mancini Racing offers a kit to go down to 12:1

http://www.manciniracing.com/ho66moqu12ra.html

Don't have to replace pitman as long as new box is same spline diameter as your existing pitman.

What would the ratio be if you used this kit with a 24:1 box?