Hi Adam,
Since you'll be in DC, why don't you try some of our great building quality East Coast species rather than importing Doug Fir/Larch 3000 miles from the West Coast.
Consider eastern pines - Southern Yellow or Eastern White.
Consider white Cedar, called Atlantic or Virginia Cedar.
Consider American Cypress, called yellow or bald cypress.
These softwoods should be excellent for replacing the fir.
Lots of people could advise you re characteristics, and point you to sources. Some here could harvest logs etc for you and even sawmill to your needs.
Keep asking and be more specific.

I look fwd to seeing your project. I learned so much on the TFGuild Frame 2004 tour to E Anglia England and meeting so many good people there.
Steve