On our Cubs the PTO end is the rear seal so make sure to get the correct one. In the M18-M20 manual, on page 11.9, it says to drive it in to a depth of 5/32 (+3/64;-1/64). I use a piece of PVC pipe to tap it into place.
The manual is free to download if you need it here.
https://www.onlycubcadets.net/forum/...ead.php?t=4648