The fun is looking at the gear calculators to see which freewheels give the nicest gear spacing and the most appropriate gears. Then comes seeing how it all fits. Perhaps a 6 will work, but some axle spacers may need moved so as to prevent the chain from hitting the dropout. Then the rear wheel will need redished and then the rd needs adjusted. And then there is also chain length.
I find the whole process quite rewarding as it gets the gearing suited to your individual situation. The factory gearing often has a bunch of redundant gears, although looking at those posts of vintage magazine reviews it seems Univega is better than most with their gearing. You are opening up a whole can of worms for yourself.