There are some DC guys here that live in DC. I googled and learned that there is no exemption on older cars. A guy failed in his 66 Mustang and the follwing is the advice he received on a blog.
The advice you've been given so far is all good. I would add a few of my own points:
HC numbers are a function of the completeness of combustion. Burn most or all of the gas, and your HC numbes will go down. Fill it up with premium fuel and drive around a bit before the test. Typically, refineries add more "oxygenates" and other additives to their premium. They'll give you a small advantage. Also make sure the car is at full operating temperature when tested. Run it for at least a half hour before the test - and not just idling. Drive it as fast as you legally can.
If they've not been replaced recently, replace ALL tune up parts including plugs, points, rotor, dist. cap, wires, air filter. As someone else noted, clean oil helps too.
Read this link:http://forums.vintage-mustang.com/vintage-mustang-forum/369921-failed-dc-emissions-inspection-66-coupe.html