18 VS 23 spline

I am honestly on this road at the moment.
I have a 23 spline tranny in my 67 dart right now.

I am thinking about an overdrive option and i keep looking at the 4 speed od from passon performance. The 4 speed from passon, as a gear set, only comes as an 18 spline.

I already have the centerforce dual clutch which works awesome. It is the od i keep thinking about.