JohnF...I gleaned the Saturn Overdrive installation data and steps from Advance Adapters' website. (These folks are very good friends and supporters of the magazine and these forums!) In the PDF instructions, you can see the relationship of parts and a reference to the torque settings for the hardware:
Advance Adapters Saturn Overdrive.pdf 722.41KB
Read through the Saturn Overdrive details and look at the illustrations. Note the similarities to your PTO sprocket location. If the bearing you describe is a caged ball type with an inner and outer race, and if the sprocket rests against the shoulder of the inner bearing race (with the bearing seated against the output shaft shoulder), then these Saturn steps should help clarify the torque setting for the nut.
Note: The Saturn Overdrive uses a stub shaft, which you do not use; however, if your sprocket attaches like the Saturn stub shaft, you'll have torque figures. What you don't want is a loose sprocket or a seized bearing!
If the sprocket seats against the inner race (only) of the caged bearing, the bearing inner race/collar can tolerate considerable fastener torque. You would not be affecting the "bearing adjustment" by tightening the nut.
I would use a properly graded castellated nut and torque the nut to spec range. The cotter pin is a safety back-up. Torqued properly, the nut will not loosen with a cotter pin in place. An all steel "top-lock" Grade 8 nut with Loctite for insurance might be an option or a similar self-locking U-joint type flange nut like this if clearance is not an issue: