Does it have a flotation bracket? If not, then it will sit really low in the stern even with full fuel. Overpowered? Based on the max HP, yes, and most likely would be an insurance issue if you ever had to make a claim, but I bet it's pretty damn fun at full throttle.
Personally I would not want that much motor on a 20', bracket or not, as they were designed for smaller 3-4 cylinder OBs of the day, particularly in the 85-115hp range. That 250 is 500lbs hanging off the back, not the 275-300lbs the boat was designed for.