[Oldsguy]

Hi Jack, welcome to our website. It is really hard to determine the horsepower of ANY engine that has been rebuilt with aftermarket parts as your 455 most assuredly is. It was common to use Big Block Olds engines in boats and most of them weren't simply stock engines. As a measuring mark though, in 1969 the 455 with the four barrel carb and all stock internals was rated at 310 bhp by Oldsmobile, this isn't the one used in the Toronado, I think it was rated at 345 but don't quote me on that. There are so many variables in horespower ratings, the big three all used different methods and it is sort of like power ratings for stereos, no real reference. There are even different ways to measure horsepower with dynonometers. It is all a relative thing, if your boat scoots the way you like it and you just want to be able to give people an idea when they admire your boat I would not be afraid to quote at least the Oldsmobile numbers. If you engine has been built with headers, a different camshaft (probably), maybe been stroked or at least bored out some with lighter rods and pistons then you are pushing 400 hp easy. Good luck and happy boating!
BTW there is a boating sub-section over at Realoldspower.com, you might check them out too!