Typically you'll get 4 to 5 years out of a battery, with heavier-duty (higher priced ones) giving longer service - I've had a few 12 volts give me 7+ years, but they were the more expensive ones that came with the cars they were in. Heavier internal plates, etc.
A good 6 volt will show something more than 6 volts, but I'm not sure of the reading. Usually the old 6 volt generators were set up to charge the battery to 7.2 to 7.3 volts - much like a 12 volt system today charges at 14.4 volts or so. Run the tractor for a while and then check the voltage - if it's at the 6.3 volt figure you mention while running then the battery obviously won't show any more than that.