What kind of boat is it? Generally speaking though the 7 gph fuel burn sounds about right, the cruise 21 gph may be a little low. With counter rotating props tracking fairly straight is no problem, but that doesn't mean hands off the wheel. Most boats pull one way or the other a bit due to weight distribution and external factors. A beam of 13' to 14' is about right for a 41, again depending on make, which would most likely be measured at the flair; not at the transom. From 1975 solid glass is a good possibility although glass over wood is also a possibility especially stringers & decks. Can't say on the weight except that boats of that era are heavy due to the use of solid glass hence the 17kt cruise. As for the motors, are they original and how many hours SMOH?