Should I Use a Splined or DD Shaft?

What is a split joint?

With DD or spline , all I ever did was add a small notch (machined) for the extra /safety set allen. No need to drill thru the shaft. IMHO ...that would provide a weak point in the shaft.

Maybe I am missing something.

I would suggest incorporating a vibration dampener ...whether you are keeping the stock box or changing to a rack and pinion. Usually on a HDK install,coming off the rack, a combination vibration dampener /universal with a DD end is used.