Please go to this THREAD and scroll down to the bottom... I need some suggestions on what to do to get some lower gearing on my Trek.
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.
Nice bike! Think I have the same year, probably my favorite all-around bike. They should've given it a longer top tube though, too much toe overlap, esp with fenders.
You mention your weight. If you're a heavy guy, I don't think I'd put a longer axle in, you're going to break it, esp. if you're loaded. Or carrying stuff.
Shimano makes 6- and 7-spd megarange FWs. the 6 is available as a 14- or 13-34, the 7 comes as a 11-,13-, and 14-34. Might be the cheapest option. Keep in might this is serious "bail-out" gearing, they all jump from 24T to 34T. $25-$40 if you do it yourself.
Second and maybe more sensible is a 110 bcd crankset. Sugino makes a really nice one, comes 46-36-26 stock. Might involve a BB as well.
If you decide to respace the frame for more gears and better selection, get a cassette wheel, preferably 135 spacing. Stronger wheel overall.
Keep in mind you're almost certainly going to need a long-cage derailleur, whatever you do.
I should've prefaced above with "How low and what are you doing with the bike?" 14-28 freewheels are easily available, and there is a good chance you keep the derailleur on there.