General questions from a new owner

The rear track I believe is narrower on those cars. They are commonly referred to as "roller skates" around here when they have the narrow wheels on them. Find yourself some wider stock rims, or mags that have a more positive offset, or just install some wider tires and it should look more even.
As for the sagging, I have used helper springs in the past, they are ok, but you can get a bouncy ride. You might try air shocks instead until you can get new springs. It's not uncommon for a leaf or two in the rear to be cracked, even on /6 cars, making them sag down real bad.
U should be able to spin the driveshaft enough in the car to grease the fittings, but replace them before you have a mishap.
Oil leakage from an old engine... get used to it I guess. If your sure the source of the oil leak is the rear main, then it's still leaking. You might check the top of the engine (valve cover, oil sender on the side, etc) to see if oil is traveling around to the rear while driving.
Hope this helps. I'm sure you'll get lots of advice here.