Ross .... After all of this, I'm coming to the conclusion that its the way Henry wanted it. Ross from your picture, the visible length of spline should be 3.35 or so. I confirmed by taking off the drive shaft and then inserting a fine long screwdriver into the end until it hit the C clip. The distance was 3 3/8.
When the mount is installed on the back, there is only 1 " of shaft visible . So from the time the yoke meets the seal, there is only another 2 .3 left before it bottoms. It seems then that my yoke must be longer than yours or an original.
Based on this I get a nominal overlap between the yoke and the output shaft of about 2.25, going up to 3.25 if it bottoms.
At the other extreme, if the yoke moves out by .75”, there will still be about 1.5” of engagement, but will be very close to coming out of the seal . Don’t like it but the only way to correct is to reduce the slip allowance
The yoke moves inwards on acceleration, and outwards on deceleration, or in reverse.