Pete, let me be the first to say, "That's awesome!"
I assume the sleeve you purchased to go over the splines is from one of the round column mills that have a quill? Something like part #7 in this parts list from Grizzly?
You're on the downhill--don't stop for nothin'!
PS A couple trivial thoughts while you're in that head:
The quill is neither very rigid nor accurate. CNC'ers benefit greatly by looking it in the up position. Most just use the stock locking handle, but you are in a position to do a little better if you wanted.
Second, I keep thinking of mounting things to the head that would be useful. For example, a broaching attachment that uses the axis motion to do the work. You don't want to mount such a thing to the spindle--it'd hammer bearings. I'd be tempted to create some mounting points on that head for such uses. I'd probably drill and ream some dowel holes for precise alignment. You could envision a variety of accessories, including that router spindle you mentioned.