Rear brake drum

I did exactly that yesterday. Tight fit ain’t it? Basically I sat down and braced the bottom of the tyre with both feet, and then thumped the top of the tyre with my hand, moved the wheel around a bit and did it again, and again, and again. Slowly the assembly went into the greased cush drive.  Good, it isn’t a sloppy fit. 

Fit the wheel without the speedo drive gearbox in place; it is a little easier than with it, but you will still need to adopt a method like Simon has described. Tighten it all up then chock the wheel so that it will not drop out of position when you remove the spindle which you should now do. Refit the speedo gearbox then spindle. 

It helps if you first smear some red rubber grease on the faces of the cush drive rubbers and/or on the paddles.

It still isn't easy, but even though the above seem a long winded way of doing it, it is easier in the long run.

Good luck!