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What cassette does it have on the back wheel? Probably a 7spd screw on. The cheapest way to get lower gears is typically to use a massive rear cassette. But you'd likely be limited by your 7s (possibly screw on) hub and current rear derailler capacity.
I'm well out of the loop on MTB stuff but I wonder if there isn't a group that will play nicely with cantis? Maybe have a look at what the CX racers use (before they all went to disc brakes).
I'm well out the bike loop now but I'm looking to sort my galaxy out so I can do some light tours again. Can you get a similar range with a modern 2x group compared to the 80s 3x7 it currently has? Would that be mtb or road?
I guess the problem is that it has canti brakes so sti's won't have the right cable pull? Please help, thanks