5.38 Swap

I just got a 71 Dart and it has a set of 5.38's in it. This car is not my main source of transportation and would be my weekend cruiser and sometimes use it to drive to work on nice days. What gears should I swap these out with? 3.73 or 3.55? Also, how difficult is it to change the gears? I know it would be easier to swap the third member out. I would rather spend $200 on a set of gears than buy a third member for almost $500. If I do buy a used pumpkin is there anything I should look for?