I just replaced a rear main seal on a big block a month ago and this is what I learned. Most if not all of this applies to small blocks too.

1. You can change it with motor/trans in place. You will need to break loose the pitman arm & idler arm to lower the center/drag link enough to get the pan off. Leave the tie rods alone & you won't mess up your alignment.
2. Oil galley plugs & rear cam seal (looks like freeze plug) can leak, but that is very rare.
3. There are 2 types of seals, 1-piece rope seal and 2-piece seal. Rope seals better but is MUCH harder to install remove. You need a "Chinese finger lock" tool to install it and a screw-in tool to remove it. Loosening the #5 main bearing cap (or all of the main bearings) might help. If using a 2-piece seal, it is stiff, so you can slide the top half up/around the top of the crank without loosening any bearings. TRIPLE check which way the lip of a 2-piece seal goes on both halves...the lip faces the engine, not the trans like is "seems" (ask me how I know)
4. For either seal, lube well with assembly lube.
5. For a 2-piece seal, make sure to offset the two halves about 1/8" so the break between the two halves doesn't line up with the break in the rear seal cap.
6. Make SURE your cap is clean & free of debris.
7. Use a single drop of special Loctite seals (check online for which one) where the two halves of the seal butt up against each other.
8. Use about 2 drops of RTV on the rear main cap surface where it butts up against the block, no more.
9. Tighten the rear seal cap slowly and make sure it goes in even.
10. Almost forgot, lube the side seals of the rear seal cap too.
11. Buy the good silicone 2-piece main seal & side seals, not the Felpro ones.

Hppe most of this helps & good luck.... and MAKE SURE it's the main seal & not the back of a valve cover or something else leaking first. Hughes Engines site has some very helpful information too.