I tried fitting the parts without the boot. Nope.
In gear. Nope.
In neutral. Nope.
Screwdriver in U-joint. Nope.
Various combinations of the above. Nope.
Even when I could align both parts by hand without the boot, and was fairly certain I had flange and groove matched, there would be no movement. It's like the receiving end suddenly shrunk by a minute amount.