Standard "rimfire" rails are narrower than Picatinny, you can get standard-width rails, or even a dual-width rail.
The only really tricky part about mounting a rail is the screws. Aside from the ease of stripping out the threaded holes in the receiver, some rails come with long screws which can protrude into the receiver, interfere with the free movement of the bolt. Lastly, use a pinhead-sized drop of blue loctite to keep the scews from backing out.
Also see the [thread=25961]ancient thread[/thread].