I was a bike mechanic in that Era and both of our bikes are from that time and I would be very hard pressed to invest that kind of money (new wheels, coldsetting the frame, possible new derailleurs etc) in a mid-range 70s bike, unless the front has a triple crankset (which was fairly uncommon back then)

It is the range of gears that matters more than the number. Both of our bikes are 10 speeds - one has a 14-32 rear cluster, the other a 14-34. With a 42 chainring on the front (which was very typical then) you are not going to get much lower.