Hi Clare,
You probably won't respond in time to help advise, me, but it's just not
going in.

I remember folks discussing putting the transmission in high gear (fifth in
this case), but I can't see why - but maybe it's to twist the transmission
shaft so that the male splined shaft at the front can be rotated a bit to
fit into the engine female splines?

I've been in neutral up until now, but I put the transmission in fifth but
what do you turn to rotate the shaft at the front?

The only thing there is are the splines on the back, but how do you turn
them? You can't get your fingers in there and you need a female spline
which you won't have.

I'm stuck. Do people normally have to rotate the transmission shaft?
How do you rotate it?