'68 Slant 225 Plug Tube Washers?

Yep, the gap should be set correctly without regard to the washers. Yep, you should remove the metal ring washers from the plugs -- go ahead and do it now, it won't take you but about 20 minutes to do all of them. If the plugs you installed weren't NGK ZFR5N (stock number 3459) you might consider swapping those in place of whatever other plug you put in. There's no such thing as a miracle spark plug, but those particular NGKs, with their extra-extended electrodes, are enough better than the standard kind to be worth it, and they cost about the same as standard plugs.