Hi Jerry,
Welcome aboard!
A close friend of the family has an 1989 41' Viking with 6-71's. He's also a seasoned fisherman that has an article coming out in a sport fishing magazine next month. I know he'd be happy to speak with you. Here's some contact details...
Randy Parrish
DParris3@bellsouth.net
850-934-1928
He's a good guy and always willing to talk boats. He's especially knowledgeable on Vikings. From previous conversations (I'm recalling from memory), he's indicated a few things about Vikings of this era that you want to consider...
Viking's hulls are 15.5 degrees. Other manufactuers, with less or more deadrise, have adopted the same over the past couple of decades, as this has proven to provide good stability, speed and a comfortable ride when the going gets rough.
Previous to 1989, Viking's were equipped with 6-71's running "dry" turbos. They tended to run hotter and produce more heat (read: more problems) until the "wet" turbo's were integrated in post 1989 era models.
Expect a cruise speed of around 21 knots, topping out around 25 knots. Fuel burn will vary with load, but generally speaking... expect around 33-35 gallons per hour.
1989 was the last year (I think!) before the style change. So if you're looking for a "late-model" looking boat, without the associated price, shop for early 90's models.
I'm recalling this from a conversation some time ago, so don't take my word as gospel. Maybe some others can chime-in here with more exact info.
Best of luck!