There weren't too many strips that could handle a DC-3, so that ought to narrow it down to several good ones from which to choose. It was my understanding that the Ford Trimotors and DC-3s were primarily used to drop smokejumpers or fire retardant, usually flying out of Grangeville or McCall. In the late '40s through the 50's they got some use as a transport for people and materials to and from the bigger strips which serviced some of the big FS camps out in remote places.
Moose Creek and Chamberlain Basin both had huge airstrips (both around 4100 ft. long) put in by the USFS to bring in big airplanes. Both have ranger stations, and both are now inside of wilderness areas. I talked to a guy named Frank Loomis that had the job of cutting up the D6 cat at Chamberlain and using a mule train to pack it out in pieces! There has definitely been a change in priorities over the years.
Stibnite is interesting due to its strategic importance during WWII supplying most of the nation's tungsten and antimony. Some good info about the strip at Stibnite in this fascinating story:
http://www.ruralnetwork.net/~yptimes/page11.htmlI was told that Ray Arnold still flies out there on a regular schedule to pick up the gold that they are currently mining.
Big Creek is another relatively long strip (3500 ft.) that was lengthened to bring in bigger planes. Here's a picture of my Dad standing near the end of the airstrip in 1941 before it was lengthened.
http://www.backcountrypilot.org/gallery/albums/userpics/12351/normal_Paul_1941.jpgIn addition to those that have already been mentioned, Landmark, Indian Creek, and Bruce Meadows are also really long strips.
It sounds like a fun project. Enjoy your research, and be sure to post any interesting stories or photos that you uncover!