Super Six fuel mileage

The 66 slant 6 in my barracuda (also 66 ) got 22.1 mpg which I felt was impressive since there are brand new trucks with modern technology that don't get that mileage. However the truly impressive I didn't find out until I tore the engine down for a rebuild and took the block and head into be inspected. My odometer says 90k but the head was in such horrible shape that the shop said it had more like 190k on it. Head needed 580 bucks just in machine work not including all the parts which was gonna be about 700 total. Ya head was in a bad way so you can imagine how that must have affected mileage and power. Motor in general was in bad shape and yet still put out 22.1. That was also city driving to boot. :) Can't wait to see what it does rebuilt. Only mods was a super six with ported exhaust n intake manifold, hei conversion and 3/4 of the exhaust was 2.25 inch with cherry bomb muffler. Rest is stock. 2.94 rear n 904.