It's not all that hard. The hardest part is getting the trans out, and even then, that's not all that hard. Take off the pipe, the roll bar, and the pipe-side TVP. Also, take the top ends of the inner shocks and the tires on the pipe-side loose (it will make getting at the screws easier... you'll see).
Next, undo the brake linkage, the four screws on the tank side TVP that thread into the trans, and the two at the back of the trans on the underside that hold it to the engine plate. After that, it's a little wedge-act and it comes right out.
Then take off the outdrives (now would be a good time to install the dual disc brakes, if you wanted to), and open the case up. Follow the directions that come with the reverse module to install it.
Also, if you wanted to do it, nows' the best time to toss in the three speed upgrade, since you'll have to take the trans out to install the bottom gear.
Re-assemble, and rip it up! Trust me, it's easier than it sounds.