"Flathead" Ford engines had the valves in the block, not the cylinder heads. "Relieving" the surface of the block between the valves and the edge of the cylinder by grinding away a little metal increased gas flow. So did "porting and polishing"- - - -grinding the intake and exhaust ports bigger and polishing the surface of the cast iron to make it slicker and reduce turbulence.
Yes, the flathead (side valve) V8 breathed very poorly in stock form, so the intake and exhaust ports would be opened up and the path smoothed by judicious removal of metal, and polishing to assist the air to flow with less turbulence - a "port and polish". The top of the block would be milled to remove metal between where the valves emerged and the cylinder, again to give more space and a smoother path for intake and exhaust - "relieving". Typically the heads would be modded or replaced too, to give more compression and better airflow, and that might include relieving above the valves as well as the relieving to the block. Bigger carbs, cam, headers and big pipes (such as lake pipes - named after the salt lakes) also helped breathing. Boring and stroking's already been covered. In a light car that could add up to something pretty quick - and 140 at the top end's pretty impressive.
Of course you could also bin the sidevalve, and put in a small block. A much more efficient design to start with, and with the same level of modification a much more powerful option.
