My advice, based on many installs (I have one in both axles of my truck, and I took all the photos in the above link):
* Screw that Downey crap. In fact, screw Downey. Their stuff is mostly junk. I've ordered from them before, never again.
* There are TONS of threads and websites out on the net that tell you how to do it. Yes, you will have to mod your axle housing to get the locker to install.
* There are wiring diagrams out there too. Some use relays, some don't. I prefer the relay method, but I ain't skeered of no electrics.
* DON'T use the gasket as a template to drill the stud holes. It's not reliable, too flimsy. Cut the notch, drop the diff in, then use an oversized drillbit to mark the hole locations. Then pull the diff, drill the holes to the proper tap size, and tap them.
* Don't bother with the longer studs for the actuator. I always use long bolts. Much easier.