I have done this many times, back in the day.
Pull the pan off and take the oil pump off of the rear main cap. Watch out for the oil pump drive shaft. I put the pump in the pan for safe keeping.
Now take the two bolts out of the rear main cap and remove the cap. Taking the old seal out of the cap side is a snap, it is the engine side that gives people head-aches.
If the old seal is a rope seal then you will need to screw in a very thin and long eye bolt for wood. Then grab the eye bolt with a screwdriver through the eye and as you pull the seal have a friend rotate the crankshaft in the same direction. It may want to fight you but just remember it is just rope so you will win the battle.
If the seal is one of the new neoprene seals do the same steps but just look out for the metal in the seal and you will get this one out too.
Replace the old seal with the newer style neoprene style they are much easier to install than the rope. Be sure the lip faces toward the engine so it will not leak.
When I replace the main bearing cap I spray the mating surfaces with some 3M High Tac spray. Then re-torque the bolts to 65 pounds feet. Install the oil pump making sure the oil drive ligns up with the bottom of the distributor. Then tighten that bolt to 40 pounds feet of torque I think. Check these specs to be sure I am correct, it has been a long time you know.