A904 tail shaft size

73 duster but I don't know if this transmission is from this car or another one like it.
Here's my problem, and this is not your problem whatsoever. I get my values a lot from the original movie The Godfather. So when someone ask me for a favor without even offering their hand in friendship or a cup of coffee (The Godfather would consider this a gesture more so than a monetary thing) I'm not real hopping eager to help. There's a section on here for newcomers to kind of introduce yourself to The Forum. This is a place you can say hello and post some pictures of your car kind of tell everybody what you have and what you know about it and what your aspirations are for it and what your current problem is now. That gives everybody the opportunity to see what you're working on to know where you're at and what you're working on and introduce themselves to you too.
So now the car being a 73 has nothing to do with the slip yoke as you said that it's not the original motor/trans.
You simply need a 29 spline slip yoke for your 29 spline output shaft of your transmission. If you're looking for someone who has experience that's and I have very little I can tell you that this is the common mopar slip yoke size.