-take that gear box apart and send that shaft in and get it splined, then have the cam hub broached ? hopefully timing can be adjusted on input ?
-Go to taper lock hub with key- it wont move- I used to work on 2' cams on 1.500 shafts, heavy load high speed, taper locks rarely failed if ever.
-Also sometimes we look at the problem instead of solution so to speak. Can you counter the cam loading to take some of the strain off the cam hub ? Generally even in high speed applications there is always some type of unloading effect as the cam goes past the high or dwell, this creates the load to be out of balance so to speak.
Just some thoughts. I didnt read too closely so maybe this was covered in past replies.
-I would not drill into the shaft or you may find the next weakest link.