I'd say for a bike of that vintage it's probably a better bet -- and MUCH cheaper -- to just swap out the freewheel (the rear cogs). That shouldn't cost you more than 40 bucks. You can get a 13 through 28 or even a 13-32, which'll give you much better hill-climbing ability.

IRD now has a line of pretty well-priced and good quality freewheels check

interlocracing.com

you can also check the Harris Cyclery site, I believe they even still have Suntour stuff in stock, and Sheldon Brown is, of course, quite knowledgeable.

http://sheldonbrown.com/harris/freewheels.html#6u

they also have a method of measuring the BCD (bolt circle diameter) of your cranks, incase you want to swap that 42 inner ring for a 39.

http://sheldonbrown.com/harris/chainrings.html

144 and 130 BCD chainrings are pretty easy to find, though I don't think you can go down to 39 on 144 BCD cranks. I think 41 may be the minimum.