I've done a couple on-the-cheap conversions with dual chain rings, and I will usually choose whichever has the best chainline. Gear ratio can be defined by your selection of cog on the rear wheel.

There are lots of chainring/spider variations out there on older road bikes, so your options will depend on what you have. take apart the chainring/spider set up and see what your options are. sometimes you can put chainrings on different sides of the spider, sometimes you cannot. sometimes you can install a single chainring, sometimes one is connected to the other. You just have to take it apart and see. choose the combination that has the best chainline and put everything back together. (if you find out you can move the large ring in towards the frame, make sure it is a few millimeters away from the stay if you decide to go that route.)

once you've done that, get yourself a cog that will give you a gear ratio you like. Most conversions like this will get you to within a couple mm's of perfect chainline.

have fun!!!