I am thinking about modernizing my 47 cm '88 Specialized Sirrus by putting a triple crank set (either Sugino or Shimano 105) on. I know this step is bordering on blasphemy, but my 40-year-old legs need more ratio range for handling hills than my 39/52 double permits. Additionally, I believe I'm currently running 170 mm cranks, but they could also be 175mm as I lack an accurate means of measuring. Is the 5 millimeter difference significant in feel or ability to spin? Also, assuming that I only change the the crankset could I get away with continuing to use my downtube front shifter? Or am I just working up a good excuse to buy a Jamis Aurora and put my Specialized Sirrus out to pasture?