All depends on what kind of shape it's in and how bad you want it! Make sure you check it for body AND frame rot. Especially check the kickup area of the frame from the rear axle back. Some Lemans/Malibus can look great from the outside but don't have enough solid frame left to hold the rear bumper up! I'd also be leery of any older car with a vinyl top, expecially in Minnesota! If she feels really 'crispy' at all under the vinyl, especially at the bottom edges of the top, I'd steer clear of it.
Yes, the '78-'79 Lemans could be had with two different V-6 (3.8) liter engines. One was the 229 cubic inch Chevy, and the other a 231 cubic inch buick.
Overall it sounds like a decent driver or possible project vehicle. If it runs and drives I'd say it's worth anywhere between $500 and $2000. The price really depends on geography though too. I know here in Michigan, pretty much any G-body, even a rotted-out crappy one, seems to have a $1,000 minimum price tag on it. I really lucked out getting my rust-free and running '78 Lemans for $500.
Either way, sounds like a neat car. Got any pictures to post? If you have any other questions about it, feel free to email me, as I have my '78. -Marc.