I am an amateur mechanic, and have installed two Mitchell Overdrive's (and man, do I love them, but that's another story).
An obvious thing to replace would be the clutch disk, but that requires removing the transmission, the bell housing, and the pressure plate, none of which are necessary for installing the overdrive (you're just near that working area when doing so).
You may wish to replace the main speedometer cable, in case your original is rough or worn.
For the first Mitchell I installed, I fashioned a pinion puller from a small, thick piece of sheet metal, and damaged the pinion bearing cup doing so. The second time I used a pinion puller tool I bought from Snyder's (made by Mitchell, I believe), and everything went much more smoothly and quickly. This is a ~$80 investment I recommend.
Beyond that, to quote 'Yes, Prime Minister,' "There are many things you can do, but nothing else you must do. You're the boss!"