Sam's right, it's not easy to do with the oil pan on. It's hard to compress the rubber gasket on the oil pan enough to slip it over the pins in the block. However it can be done as I needed to do this and wasn't about to take the block out of my car. Here is a cut and paste from a conversation I was having with a nice fellow named Bill from the chevelle.com site. Motor is a big block, not sure if small block is exactly the same.
I assume that you have already removed the timing chain cover, correct? You can reuse the rubber cross seal however if you have already attempted to get the cover back on you'll note that it is very tricky. That seal is hard to compress and makes for a real battle getting the cover back inplace. What most guys do when replacing the cover with the engine in the car is either grind about 1/2 the trailing lip of the seal groove off ( the edge that will be inside the engine) which lets the cover get over the seal with out displacing the seal itself.There is still plenty of "groove" to hold the seal in place. Another " Preferred" method is to get the cover as close to installed ( On one locating pin ) and then take a slim "drift" and put it through one of the holes in the cover, on the other side of the cover, and get the small end of the drift into the corresponding hole in the block and pry down on the cover until you get the cover on the other locating pin. Equally as difficult. It's tough to put that cover back on with the oil pan in place. There is nothing easy about doing that in car. If you have the engine out of the car I would remove the pan, install the cover and replace the pan. That's easy. Finally I ALWAYS put some silicone on the mating surfaces between the cover and the block. I also put silicone on the mating surfaces between the block and the oil pan. ALWAYS! Just work it into the mating area with a finger.
Ok, cut and paste is over. I used method 2 with the drifts prying down on the cover. I cut off the two nipples off the rubber seal at the edges, and found I needed to be able to push the cover in when I'd lined up the second pin. I used a little piece of wood I could get on the cover by the pin, and then just hit it a bit with a rubber hammer. I also did about 8 dry runs without any goo until I got the hang of it.