Yes, only 418 heddles to thread. I have added eight threads on each edge in 2/2 twill in hopes it will make it so I do not have to use floating selvages. With a pattern repeat of only 62 ends, maybe this will go fast!! (famous last words?)
The traditional blocks for overshot are: 1-2, 2-3, 3-4, 1-4. As you can see, each block shares a shaft with it's neighbor. The blocks can be as wide as you want, as long as the floats are not to long for the intended project. There are different ways of treadling overshot: rose fashion, star fashion, or on opposites. Or, if interested in creative overshot you might want to try and find Shuttle Craft Guild, Monograph Thirty One, by Margaret B. Windeknecht: Creative Overshot.