I feel like this is the stupid question of the day, but how do I change the fluids on a 1971 10 bolt rearend. My other cars have always had a fill plug.

I have assumed that I would:
1. drain the fluid by removing back cover
2. replace gasket and reinstall cover
3. fill with differential fluid

Here are my questions:
1. should I use a gasket, RTV sealer, or both?
2. Do I put the fluid in through the vent hole? if so, any trick to removing the vent (I think it is a vent)?
3. where can I get a replacement vent.
4. How do I know how much fluid to put back in?

As always I would also appreicate any good advice on this seemingly simple task.

Jay