Hey Jim, It sounds like you are thinking about getting a planer! Yes you are probably right about the amount of board you need to attach runners to. I have not made a dedicated sled for this because I usually don't have many short boards to plane. I have done the board butted up against each other, but you have to be fast and accurate. It is a tough thing to do, but It can be done.
It is important to take off equal amounts off both sides. I have seen boards cup and twist because of uneven moisture levels on the board. Your best bet is to try to plane off the back side first, then take a little off the front. You will probably end up with about 1/2" thickness, and if your lucky, the board will stay flat. This is why the few boards of finished flooring stay in my scrap wood box.