You're probably better off asking in the mechanics forum, but here goes. Ebay and Nashbar are where I look for oddball parts. I think something like this might fit the bill:
http://www.nashbar.com/profile.cfm?c...eid=&pagename=
The "shift ramps" they talk about will make shifting a lot smoother. The biggest gear they have is 28 teeth, which is what a modern mountain bike typically has. The problem you might run into is that your derailleur is not long enough for such a big gear. Nashbar also has this derailleur
http://www.nashbar.com/profile.cfm?c...eid=&pagename= which is less than ten bucks, accepts up to 34 teeth, and meant for six speeds.
I'm assuming from the vintage of your bike that it doesn't actually have a cassette, but has a "freewheel." Here is an article about freewheels which shows the difference:
http://sheldonbrown.com/freewheels.html
I would expect that a 70's era bike would shift dramatically better with a more modern derailleur and gears.