Re: Wellcraft boats
I have a 91 Wellcraft 216XL Eclipse cuddy cabin. Good, solid boat with a very nice ride. Handles rough water really well for it's size. We ran from Port Washington, WI to Milwaukee, WI on Lake Michigan in 3-4+ foot waves. Maintained 30 mph without a problem.
They have pretty deep V, 20?, so more power is better. I'd say at least the 5.7l, but the 7.4l is KING! Makes it a real sleeper, not all that many 60+mph, 21ft boats out there.

I wouldn't worry much about the fuel consumption. There isn't that much of a difference with the bigger engines. We averaged 2.4mpg for the season and next year it should get better with a higher pitch prop.
When we were shopping around we found the Eclipse to have more room than other 21ft boats, especially in the cabin.
If you goto http://www.wellcraft.com/ and click the Owners link, you can get the product info guides all the way back to 1985.