Just a comment, pictures are deceiving.
Be careful the shaft increase in size does not get larger than the root of the smaller splines.
You may have to work on that also.
Yes. New "small diameter is 1.22" dia and minor dia of splines is right about 1.300.
Snap ring dia are about 1.286".
Rob, If I were you, I would talk to Winters about this first, heres why;
I am not going to dig up formulas, only share what I see.
The main shaft reduced section is to absorb the torque in a place away from the circlip grooves.
The reason for this is that the circlip groove is very short and makes a very concentrated stress riser.
The difference in size between your new shaft and the groove is only about 0.033 per side. I may be wrong, but I would expect the shaft to break there next.
Now, if there is a way to eliminate the circlip groove, you could get the larger dia to live.
Best of Luck and keep us informed, John