One problem. Even the newest "EAB" and "EAC" heads are over 70 years old. Unless you find a truly untouched engine and remove them yourself, you can never be sure that the heads have not been resurfaced or otherwise modified. Thus, most of the time you can never be sure of what you have. In my opinion, someone putting another set of heads on an engine without knowing where the clearances are may be making a big mistake. In my experience, optimum combustion chamber turbulence (quench) yields more benefit interms of performance than the increased compression.
The first flathead I built over 30 years ago was a teenage magazine-readers dream : a 276" merc with a Holley 390, an MSD ignition, and a set of new Offenhauser heads. I just took the heads out of the box and bolted them on. Between the SBC ignition curve in the MSD and the obvious lack of combustion efficiency with the unmodified heads, I was never really happy with the way that engine performed. I ended up selling it (and the '36 3-window it was in) after 5 years. I have a 255ci Merc with carefully fitted Edmunds heads, a small base 2G, and a Mallory dual point in my current '51 club coupe. I am very happy with the way it performs and believe that the '51 would outrun the '36, even giving away 400 lbs. and 21 cubic inches.
