318 MAX fuel economy builds?

I haven't gotten 20 with my 273 or my Teen 2 barrel but I can get 18-19 pretty steady. I run a 3.23 gear and 255-60 15 tires. I have a LD4B and a AVS2 for the 318 that I might put on one of these years. I might be able to get a little more performance and not sacrifice highway mileage.
I still think the key to good mileage is to keep the cruise rpm below 2000 for a stock engine.