NP 231 Input Spline Swap Questions

I have a NP 231 with a 21 spline input and I am planning on putting it behind a 91 AX15. There are several 23 spine inputs out there and I am wondering if anyone knows exactly which one will work. Factory part numbers or Crown part numbers would be the best. Thanks in advance.

This should give you everything you want to know about.
http://www.novak-adapt.com/knowledge/np_nvg_input_gears.htm