I have a 1970 Chevelle SS with a 1969 427 and am about to replace my rear main seal. I have a good size leak coming from the Bell Housing area and am 99% sure it is my rear seal. I have replaced the valve cover gaskets and tightened everything else up as well so am pretty sure the rear seal is bad. This is my first time replacing one, can someone please give me some guidance on replacing one? Such as tools needed, etc. I do know it is a two piece and should be able to get to it by dropping the oil pan. Thanks!

Well first of all are you really really sure its the rear main?
It could be the oil pan gasket. Never the less
You are going the need a pan gasket any ways, at that point you might as well do the rear main.
I wont kid ya its a pretty big / time consuming job. For the amount of work it takes to get the pan off as well as get to the rear main with the motor in some will say you should yank the motor out.

Why you ask? Well because its going the difficult to get the pan out in the car. At the least you are going to have to pull the connector bolts for the motor mounts an get the motor as high as you can and then need to shore it up for safety and still need to be able to get around the under side of the motor. Pull the starter then fight getting the pan off. this will be more difficult if you have hedders.
If you get that far then you have to scrape all the old gasket material off the pan (Easy) and the block which can be hard as a brick (not easy) with very little room to work. Then pull the oil pump, rear main get the old gasket out. Again with very little room. You have got to get the surfaces super clean. That will be a key element to have no leaks when finished.
If you have not done this before it will be easier to screw by ending up with an oil leak after you did all above and put it back together.
So what I am saying is it might be easier to yank the motor because with it out it will be a hell of a lot easier to get the pan off and change the seal maybe install a new oil pump successfully.

If you are going to do it in the car A cherry picker would be very helpful. You will need a set of 4 quality jack stands a sturdy jack probably a couple of bottle jacks as well as another floor jack. A full set of sockets and or a set of wobbles, a set of wrenches, extensions, ratchets, rubber mallet, pry bars and a lot of Asprin :D

I my self would do it in the car because if you get it right, which I would, Its simply less bolts to take on and off doing it in the car.

But If you want to get your hands dirty and learn how to do it it would be a great learning experience. While the motor was out you could clean up the engine bay real nice.
Just take you time and if you have problems we are all here for you.

If you dont have any tools or experience as well as patience maybe you should have a pro do it.

Very good advice!!!