Stock rear gear in a '73

What do you need to know?

See, I am considering upgrading my rear gear and would like to get power to both wheels. But I would like to know what I have now so I can get a starting point. Is there a way to figure out what rear gear I have?

What gears were available in '73, stock?
 
I got it! I had a 6 on the Data code plate so that means I have a 3.00 standard gear.
What kind of difference would I feel if i installed a 3.50 locking differential?
How much impact would there be in my MPG? I do alot of highway driving.