[dd74]

To OP: it is possible to go with a wider (nee larger) gear set in back, but that will may entail changing to a longer rear derailleur and chain. But parts from the downtube shifter era are hard to come by; you might be looking around for a while. At any rate, Bike Forums.com has a very good vintage site you should check, and pose your same question as here.

Personally, I would not upgrade your bike much further than a gear set, RD and chain, because at that point you will most likely be in for much of what a new bicycle costs. These days, you can probably get a newer, more efficient, lighter bike with modern components that is easier to live with for much less than a $1,000. Check out the major brands, (Trek, Giant, Specialized, etc), and I'm sure you'll find something to your liking.