What type of block is it? If it is, as I'm assuming, a chevy, then it should have a 2 piece rear main seal. I that case, all you have to do it get enough clearance do drop your oil pan, remove your rear main cap, remove the seal (be careful, don't jab anything, or you'll knick your crank, and then you will have to pull it!) and put a new one in. When you install the top side seal, be patient with it, and also put a little oil on the seal so that it doesn't start up dry, tearing up the seal.

 

Off set the seal gaps, so the gaps don't maych with the main cap gap. Also go back with a one piece oil pan gasket. I belive felpro has them.