If the fish are shy, running your spoons far behind the boat may get you action, but I think 300 feet is farther than necessary. If the fish are aggressive, they'll hit a spoon right in your wake, on the surface, six feet from the back of the boat.
Spoons fished far behind or on boards are a stealth presentation. Flasher/fly combos (or dipsies) are the opposite - they catch a lot of light and put a lot of vibration in the water. They don't have to be far from the boat to be effective (but still can be).