The shifters look like friction only. If they are get a frewheel with a lower low gear, looks like you have a 25t. There are not a whole lot of choices with five and six cog freewheels so you might also need to respace the frame to 130 and put a wider axle in the hub, in order to use a 7 0r 8 freewheel. In the front if you went triple you would need a new crank, BB, der and rings. So to save money, if you have a 130 bolt circle, go get a 38t chainring, that is as small as you can go. If you have a smaller bolt circle you might be able to find an even smaller ring but sizes outside of 130, 135 and 110 are uncommon. Fortunatly you live in NYC and I would bet there many out of the way old bike shops that may have the parts you are looking for.
While looking around, keep your eye out for a triple bike the same era as yours, but crashed and cheap. Buy it and swap parts.
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