Yea, it's possible to get 20 or better mpg's out of your 360 though a high performance mill is not going to be. Some of the things I didn't do that are supposed to help get you mileage I didn't do. I wasn't hunting for mileage. The best I ever got was 22 on the Hwy. while traveling from New York down to Ga. Gasoline seems to be better off the Long Island. Cheaper and a better bang for my gallon. But to say I would get 22 is not accurate. 20 was the best I could achieve on a regular bassis if I drove it easy. (Speed limit or there about. Not 70+mph!)

At the time, I needed to have cat's on the car. I didn't want to use the standard 1 size fits all headers that are going to get wrecked on the first speed bump or pot hole. Dougs and TTI's were to much for my pocket. But I think headers would have helped just a little bit. My exhaust was 2-1/4. Twin cats and Thrush Turbo mufflers connected with an H pipe done at a muffler shop. Considering the low power of the '79 mill, I'm not to sure if a 2-1/2 exhaust would have helped.

The Fed-Mougal 625 Carter carb was well tuned. I would have liked to try a TQ. The idea of the smaller primary for a higher velocity charge. Good for atomizing the fuel. Good on low rpm mileage hunts. The perfect match for the factory iron intake. Which there pretty darn good intakes. I did think about trying 2 other intakes though. A Performer and a LD-340. The Performer would have added velocity. The LD-340 is a high rise, large port via compare to the Performer. The Perdormer for high velocity and the LD-340 for having the large port for the bigger C.I.D.
I'm still not sure which one would be better.

With the low compression of the engine, I could not see installing a set of closed chambered heads or a cam. (More of a money issue at the time.) The size of the cam would not be to large. Since mileage was the idea. Just a split duration cam to help. The idea of getting a better cleaner intake charge. A few more degrees on the intake May actually help. As long as there is an increase in torque in the rpm cruising range, mileage could increase.

Most everything is in the tune. Not just the carb but also in the distributor. The whole engine must be looked at and maxed. Then of course, the rest of the car comes into play. Balancing the tire size with the gear ratio in the cruising rpm to keep the engine rpm at or in a high torque area of the engines power range.

I'm not sure if the 904 I had was a lock up. But a LU is known to add a little bit of mileage. I only gave the trans a deep pan.

(As stated earlier, I didn't have the funds to tinker around. Which I love to do. House & family are first, tinkering & toys later.)