hello from New Zealand, 18 spline head ache a833.

I believe either that was GM or some other adaptation. YOU NEED TO MAKE CERTAIN this is not an overdrive box. To do that all you need to do is to determine if 3rd gear is underdriven or overdriven. Take a wrench and rotate the front shifter shaft clockwise which will put the gearbox in 3rd gear. Now rotate the front shaft exactly 1 turn while watching the output shaft. If it goes less than 1 turn it's a conventional 4 speed. If the output goes more than 1 turn it's an overdrive box.

This thread has a couple of good dimension charts
https://www.forabodiesonly.com/mopar/threads/cutting-a833-input-shaft-question.531577/