After you wiggle out the speedometer cable's end piece, you'll be thinking, "How da heck am I gonna get dat big ol' thing through that little bitty hole?"
Be sure the speedometer drive fitting and cable area are clean. Warm up the new rubber boot and get it as pliable as you can. Liberally grease the boot both inside and out then start working it over the cable fitting. It will -eventually- go over.
Before refitting to the gearbox, pack the inside of the boot with grease. Check the anchor bolt... it's gun-drilled (hollow) and should not be clogged. Use a short 10mm wrench to snug up the bolt. No gorilla torque as you can easily strip the aluminum threads in the gearbox. Argh! Don't do that.