Whenever I'm stumped with questions like this, I go to the parts diagram for maybe another explanation. In this case, the parts diagram shows the tach gear as being installed after the cover has been installed. If the tach gear went in first and then the cover, the parts diagram would show the tach gear below the cover. In this case, the tach gear gets engaged in the spigot in the cylinder head and trapped in place by the Tachometer gear guide item#9. So you'll want to install the cylinder head cover without the tach gear first, then place the tach gear in place then install the tach gear guide.
Woke up early, 0400, thinking about this. Opened the FSM digital version and
Then I open this forum and see my friend and truly smart feller has already researched and beat me to the punch.
There's a Lesson Learned here.
Don't think you know it.
RTFM first then proceed.
I've pulled this cover many times, don't recollect every removing the entire assembly first, but than again it's been at 15 - 20 years since doing so. In other words an 'infrequently performed evolution' using nuclear parlance.
In this case, I got lucky. Very lucky it didn't fall out and into engine internals.