Spline?

I currently have a slant 6 72 Duster. I did a front disc brake conversion two years ago, but I'm still using the old 5/4 bolt pattern. I am very eager to switch over to the 5/4.5 pattern, but the rear end is limiting me. I don't have the coin to completely change out my rear end at this point. Is it possible to just swap out the spline? I'm not too knowledgeable in this area and any help would be appreciated.

What is the number that is associated with the splines? I see 28 or 31 spline for example. What measurement is that?