I believe there year they changed from taper mainshaft t I Spline was '87-'88.

If you wanted to change yours to a spline shaft, you're hmgonna need to change the inner primary, transmission, or have your trans, machine out the starter mounting bore for the newer style starter to match up with the ring gear on the Spline clutch basket. That's a lot if work unless you want to get rid of the old "obsolete" parts for the newer and still available parts.
I have the opposite problem, I want to find a Taper Mainshaft Baker 6-Speed gear set for my '84 FXR, THATS MY PROBLEM.
As Baker doesn't manufacture the Taper Mainshaft gear set anymore.

 

Remember...If not pointed out..There are most certainly two different Tapers...The Original , which is Basically a Morse Taper...and the Late '84+ which is Decidedly Not..in fact...I consider late '84+ the Very worst Possible alternative..Give me the good ole Shovel clutch, or, of course, Splines!!

 

I’m following this for general knowledge. This post is especially interesting because I have what is ( apparently) the worst possible alternative..🙀. 1985 taper w/ key. For years I’ve been envious of spline-shaft people and thought to myself… someday I’ll make the change. This thread makes that unlikely at this point in my life..🙄. Seems a lot more involved than I thought.

 

If your late '84 has Not given problems...be Thankful and do Not Fool with it!! If disassembled reassemble carefully, by The Book, using All original parts.. Once that clutch hub goes South..I pitch everything, including Shaft..Replace with Good ole Shovelhead Shaft and Clutch, and live happily ever after... Each of my boys has one,, we treat them Very Carefully.. Come to think of it...I still Have One.. it is Currently Functional..

 

My ‘85 ( don’t know how “ early or late it is) is currently on clutch hub #4 in the 327,000 miles I’ve ridden her since buying her. new in ‘85.
Hope she holds out ‘til I reach the stopping point a lot of guys get to, if they live long enough.