Since you are intending to get a new shaft made up, and several guys have mentioned that the front splined yoke was a bit short, (as is mine) I recommend that you measure from the face of the splined male power output shaft on the belly PTO to the face of the male splines on the carrier under the tractor. Mine could be longer as well, to get more "overlap" on that slip joint.
Be aware that the location of that shaft in the carrier can be set at different lengths, depending on where it was when the set screws were tightened. My Installation Instructions called out that dimension, so I would check that the protrusion is correct before measuring.
All these words to say, " make the new shaft as long as possible and still allow it to be connected."
It seems to me that to get the most overlap in the slip joint and still be able to attach the rear-most yoke to the belly PTO, that the front, long slip yoke would be slid onto the carrier shaft until the (rear) nose of the carrier shaft contacts the cross of the universal joint. In other words, shove it on as far as you can, then calculate your desired finished length. I would allow (subtract) about 1/8 inch of "collapsed" length to make sure I could attach the PTO end of the shaft.
Seems this procedure would result in maximum engagement of the slip yoke and the male splines, but still allow hooking and unhooking the driveshaft.