If it's the 4.3l V-6, it is literally a 350 sans two cylinders. The V6 is going to have to work that much harder to move that big car around, so if you're looking for fuel economy you probably won't see an improvement. Keep the 350 and look at an OD transmission. If you're just looking for less power, you're going in the right direction.
1968 Impala SS