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
Well the 153 cube four cylinder "Iron Duke" that powers Ag water wells and fork lift trucks as well as the refrigeration unit slung under Semi trailers bolts right up as well.
GM used engines bigger than 400 cubes only in the their full size cars all other cars in GM's fleet were restricted to 400 cube or smaller.
This is why Caddy had a 500 cube motor, while Buick, Olds, and Pontiac had the 455. Chevy at the time was still running a 427 in full size cars up to 1970 when the 454 was introduced to move the heavy full size Chevy.
I used to work on a fleet of twelve Biscaynes powered by the Iron Duke four cylinder. The cars had no radio, and no heater bench seat and manual three speed tranny. These cars were purchased by the water department of the city of Tampa. These cars had a hard time exceeding 42 mph.