Due to the nature of the flathead and the length of the runners that deliver air from the valves you have a considerable volume. By reducing the height of the gasket or gaskets and even going to a copper shim (popular in the old days) you will only gain a tiny bit of compression and the down side is that you have now reduced the area restricting the air flow both in and out. It is a fine line working with flatheads to balance the intake/exhaust versus compression required for more dramatic cam timing. An overhead has the runners behind the valves reducing the volume (being compressed) considerably and the I&E can be opened for higher volumes of air.
A side note I know of a few old Ford flatheads that have replaced the small blocks that have occupied their frame rails. There have been some vast improvements for flatheads going to roller type tappets and cam profiles. There are a few 32 flatties on the street that are quite fast today.
Dave Neptune