Nothing says you have to upgrade the rearend when you change engines. If there's enough HP and you don't have a good rear axle, then you need a good one, but otherwise it's not mandatory.
The gears go in the third member (center section) of your rear axle.

 

o ok, well im sure ill find all this out in school. but i kinda want to get a base knowledge now

 

I personally like this rear gear/speed calculator. You can put in all the gears for your transmission along with redline and tire size and it will tell you the top speed for each gear. You can also put in another transmission/tire size combo and see the difference between the two. It even does a really nice little graph.

The presets in the link above were with my old TH400 transmission on the right and a T-56 on the left with different rear gears and tires.

You can always just throw a T-56 in there and have the wonderful world of two overdrives.

 

Upgrading can mean either higher or lower top end speed, depending on what ring and pinion you use. As I mentioned before, the ring and pinion are in the center section of your rear axle.
The higher the numerical number on your ring and pinion gears, the lower your top speed. So if you had 3:08 to start and went to 3:73 the result would be quicker acceleration, but lower top speed.

 

im pretty sure all i have is the stock ring and pinion for the 79, i think its a 2:73. wat would be a good one to switch to if i were to rebuild my engine to 450 hp?

 

How did that 6 speed swap go? Are they as heavy as they look? If I can find one for cheap enough I'm gonna skip right over the four speed and set this thing on kill.