You need to pull off a valve cover and see what injectors are in it. There will be a round tag on the right front of each injector which tells you the size is which is one of the clues to tell you your horsepower. The serial number is worthless after it leaves the factory, unless you know nobody has made any changes.
If everything is the same as the day it was born 330HP is a good guess for a highway coach unless is was built for transit use which would set it back to 277HP.
