Slants are super durable and reliable. They have good low end torque, but HP numbers are not impressive. A well tuned /6 can get 17-18 mpg around town and 22-24 highway mpg. '73+ come with electronic ignition, earlier models are easy to convert, valves need yearly adjustments. Slantsix.org has TONS of helpful slant owners. Many recommend running larger exhaust for improved mileage.
Changing to a super six manifold and 2 bbl carb is popular, try to get new carb linkage if you can. However, from a strict mpg standpoint you not likely to gain a lot, so don't spend a lot to make the change. I like my single bbl just fine.