Looks like a Kent, WA Raleigh, if so, threading will not be a problem. Buy local, as you will likely need advice, and the ability to take back stuff as you are looking at some trial and error. If you just add a granny gear to that crank, remove it first, take it to your favorite shop, and get their guidance. You will need to change BB. And I have seen some cranks that were "almost" set up for a third ring. Crank arm would have the bosses on the back side for a third ring, but they were not tapped and threaded.
FWIW: I put a modern compact crank on a couple of vintage bikes. It will give you quite a bit of additional gearing. Is it enough? Only you will know. Its all about motor.
Run the gear calculator on Sheldon Brown site. A swap from the 39 to a 34 (common compact crank) gives you about a 15% change in bottom gear.
Again, bike shop can guide you on bb length. Online bike parts buying = hassles when stuff doesn't work out. Best suited for consumables and spares.
Last edited by wrk101; 02-05-12 at 05:44 PM.