The flathead always has a Hobson's choice, striking a balance between breathing efficiency (large X-section passage from the valves to the bore) and static CR.
The best compromise depends not only on what the engine is used for (maximum torque vs. peak power are different), but the basic architecture including bore to stroke ratio. Unlike OHV, a small bore + long stroke engine has an advantage because the small piston makes the total chamber area narrower without reducing valve size, and the long stroke adds displacement and compression without shrinking the transfer area.
AFAIK the power king of all flathead production is the last H-D factory KR in 1969:
2.745" bore by 3.8125" stroke (the basic engine was introduced in 1929) = 739 cc, 45.12". The best ones produced 58 hp on pump gas at 7,500 RPM from 5.5:1 CR and really big cams.
In a 292" Ford flathead (seriously big, bored & stroked) that would be 375 hp. Missed it by over 100...
