I just had the same issue with mine..had about an inch to go and, no dice. Spoke with my omc mechanic and he said, if everything is greased in there, engine is aligned, kick it in! I was a bit shocked but did what he said..I turned my foot full horizontal and gave a firm kick just in line with the gear driver, near the top and woosh, it just slid in, no harm no foul...just needed a bit of assistance to get it past a sticky spot.
When the stern drive splines alight to allow the unit to slide all the way in, but vice grip pliers on the prop shaft (or the prop) and turn it a bit while pushing the stern drive in. You’ll feel things line up and the stern drive will slide in easily. Yo7 never want to force it.
One last question on this: after watching this video over and over, I believe that my issue might be from the miss-alignment of the motor. Any tips on how to determine what way to adjust the motor mounts? Have you found any place that one could measure something to see where the alignment could be off? Or is it all by trial and error?