My wifes aunt gave me a 1981 Buick Riviera that she got new and kept in great condition.It has the 307 in it and I find it to be way to much of a gas hog.does anyone know if I can swap the V8 out for any one of the 231 V6'*.

Gas mileage would be just the same or worse due to the fact that a smaller motor will work harder to move that car ...i would swap the carb for a edelbrock 600 instead of the stock Gm

Do you think I should change the intake or use the stock one?

If you travel a lot of highway miles swapping the transmission for a 200r4 will help milage, you won't see much benefit driving round town though.
Installing a properly calibrated fuel injection system will benefit overall mileage.
In both cases (or changing the carb/manifold) you will have to travel a lot of miles to save your initial outlay. If you only occasionally use the car it may not be worth the cost, plus the car won't be original anymore.

I forgot to add, I had two Gbody wagons, a '79 Buick Century with a 231 v6, the other an Olds cutlass with a 307 and lock up converter. I got better mileage from the Buick (24 as opposed to 20 respectively). Keep your car in tip top tune, that will help mileage whatever you drive.

like rodger said give her a good tune up and make sure everything in the quad is working properly, good street performance ie. mileage and throttle response is what the quad was designed for. thats your most cost effective thing i would say