Johnny interesting motor you have! The Daytona Marine 409 conversions in early 60's were used by the early offshore racers as they were pretty heavy hitters back in the day. They did make regular lower horsepower conversions like several other marinizing companies did. The base horsepower was 280 from the 409 blocks and yes they were notched. The Daytona 400 was a twin turbo unit and from what I recall were just under 400 HP but possibly some were rated at 400 HP. I am assuming the exhaust system and turbo's from your's are long gone.
Robert
Robert